Ethanol-Induced Depression: Exploring the Underlying Molecular Mechanisms

Abstract

Ethanol consumption is widely recognized for its detrimental effects on mental health, particularly its association with depressive disorders. This narrative review aims to explore the intricate molecular mechanisms underlying ethanol-induced depression, synthesizing findings from preclinical and clinical studies. We begin by providing an overview of the relationship between chronic ethanol consumption and depression, highlighting compelling evidence from diverse populations. Subsequently, we delve into insights from animal models that elucidate the pathophysiological changes triggered by prolonged ethanol exposure. Key mechanisms identified include oxidative stress, which contributes to cellular damage; neuroinflammation, characterized by the activation of glial cells and altered cytokine profiles; and disruptions in neurotrophic factors that impair neuronal growth and survival. Furthermore, we discuss the induction of apoptosis in neural cells and the significant impact of ethanol on neurotransmitter receptor remodeling and regulation, leading to altered synaptic transmission. While substantial progress has been made in understanding these mechanisms, we also acknowledge the limitations of current research methodologies and call for further investigations to translate these findings into effective therapeutic strategies for individuals affected by ethanol-induced depression. This review ultimately underscores the need for a comprehensive understanding of the molecular underpinnings of ethanol’s impact on mood disorders, paving the way for improved interventions and preventative measures.

Graphical Abstract

Introduction

Ethanol, commonly known as alcohol, has long been recognized for its impact on mood and behavior, serving as both a social lubricant and an agent of escapism for countless individuals. However, its darker side, particularly its association with mood disorders like depression, has come under increasing scrutiny in recent years (Hedayati-Moghadam et al. 2024). The relationship between ethanol consumption and depressive symptoms is complex and multifaceted, spanning both psychological and biological dimensions (Keyes et al. 2019). This narrative review aims to delve into the intricate interplay between ethanol and depression, focusing on the underlying molecular mechanisms that may contribute to this relationship.

The prevalence of alcohol use disorders (AUDs) and the high rates of co-occurring depressive disorders are alarming statistics that highlight the public health implications of ethanol consumption. According to various studies, individuals with major depressive disorder (MDD) are more likely to engage in excessive alcohol consumption. At the same time, those with AUD often experience symptoms of depression (Kakanakova et al. 2020). This bidirectional relationship raises critical questions about the nature of these connections: Does ethanol consumption lead to the development of depressive symptoms, or do underlying depressive states predispose individuals to alcohol use? Understanding this interplay requires thoroughly examining the neurobiological and biochemical pathways affected by ethanol.

At the molecular level, ethanol exerts a wide range of effects on neurotransmitter systems, particularly those involving gamma-aminobutyric acid (GABA), glutamate, and serotonin. GABA is the primary inhibitory neurotransmitter in the central nervous system, and ethanol enhances GABAergic transmission, leading to sedative effects and relaxation. However, chronic consumption of ethanol can result in a downregulation of GABA receptors and altered GABA signaling, which may contribute to anxiety and depression (Li et al. 2020; Ali et al. 2024; Nicosia et al. 2024). On the other hand, ethanol’s effects on glutamate, the principal excitatory neurotransmitter, are equally significant. Ethanol inhibits NMDA (N-methyl-D-aspartate) receptor activity, which can disrupt synaptic plasticity and impair cognitive function (Adell 2020). These alterations in neurotransmitter systems are further compounded by the influence of ethanol on serotonin pathways, which are critical in mood regulation. Chronic ethanol exposure can lead to dysregulation of serotonin receptors, potentially exacerbating depressive symptoms.

In addition to neurotransmitter systems, ethanol impacts neuroinflammatory processes that may play a role in the development of depression. Research has demonstrated that excessive alcohol consumption is associated with increased levels of pro-inflammatory cytokines and activation of microglia, the brain’s resident immune cells (Mikhalitskaya et al. 2024; Ruiz et al. 2022). Chronic neuroinflammation has been linked to the onset and progression of depressive disorders, suggesting that the inflammatory response elicited by ethanol may serve as a bridge connecting alcohol use and mood dysregulation. The relationship between neuroinflammation and depression is further complicated by the effects of stress, which can amplify the neuroinflammatory response in the context of alcohol use (Mikhalitskaya et al. 2024; Ruiz et al. 2022).

Another critical aspect of ethanol-induced depression is its influence on the hypothalamic–pituitary–adrenal (HPA) axis, which plays a crucial role in the body’s stress response. Chronic alcohol exposure can lead to dysregulation of the HPA axis, resulting in altered cortisol levels and increased stress sensitivity (Dunne and Ivers 2023; Nikbakhtzadeh et al. 2023). Dysregulation of the HPA axis is a well-established feature of depression, and the interplay between stress, alcohol use, and mood disorders underscores the need for a comprehensive understanding of these mechanisms (Stephens and Wand 2012). The impact of ethanol on neuroendocrine function highlights the bidirectional nature of this relationship, where alcohol use exacerbates stress responses. In contrast, stress may drive individuals toward alcohol consumption as a means of coping.

Furthermore, genetic and epigenetic factors must also be considered when exploring the molecular mechanisms underlying ethanol-induced depression. Genetic predisposition plays a significant role in an individual’s vulnerability to both AUD and depressive disorders (Kaminen‐Ahola 2020). Variations in genes that regulate neurotransmitter systems, neuroinflammatory pathways, and stress responses can influence an individual’s risk profile. Additionally, epigenetic modifications induced by environmental factors including alcohol consumption, may alter gene expression patterns associated with mood regulation and stress response (Wedemeyer et al. 2020). These genetic and epigenetic dimensions add layers of complexity to the understanding of how ethanol influences mood.

Behavioral and psychological factors also contribute to the nexus between ethanol and depression. Many individuals consume alcohol as a means of self-medication, attempting to alleviate feelings of sadness or anxiety (Hallihan et al. 2024). However, while alcohol may provide temporary relief, it often results in a vicious cycle where increased consumption leads to heightened depressive symptoms. The psychological mechanisms involved, including cognitive distortions and maladaptive coping strategies, further complicate the picture, making it essential to consider both the biological and psychological perspectives in this context.

In summary, the relationship between ethanol and depression is a multifaceted issue that encompasses a myriad of molecular mechanisms, neurobiological changes, genetic factors, and psychological dimensions. Understanding these intricate connections is crucial for developing effective interventions and treatment strategies for individuals grappling with the dual challenges of alcohol use and mood disorders. As research continues to unravel the complexities of ethanol-induced depression, it is imperative to adopt a holistic approach that addresses both the biological underpinnings and the psychological factors at play. This review aims to provide a comprehensive overview of the current understanding of the molecular mechanisms linking ethanol and depression, setting the stage for future research and clinical applications in this critical area of mental health.

Chronic Ethanol Consumption and Depression Disorders: A Review of the Evidence

The relationship between chronic ethanol consumption and depression has been extensively documented in epidemiological, clinical, and neurobiological research, forming a cornerstone of our understanding of alcohol’s dual role as both a self-medicating agent and a neurotoxic driver of mood disorders (Gao 2022). Epidemiological studies spanning decades consistently reveal a bidirectional association between heavy alcohol use and depression, with each condition exacerbating the severity and persistence of the other. For instance, meta-analyses of population-based cohorts estimate that individuals with AUD are 2–3 times more likely to develop MDD compared to non-drinkers, while those with baseline depression exhibit a similarly elevated risk of transitioning to hazardous drinking patterns (Gao 2022; Aslam and Kwo 2023). This interplay is not merely correlational; longitudinal data from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) demonstrate that chronic ethanol consumption—defined as sustained intake exceeding 80 g/day for five or more years—predicts the onset of depressive episodes even after controlling for confounding variables such as socioeconomic status, genetic predisposition, and comorbid psychiatric illnesses (Hasin and Grant 2015). These findings are bolstered by clinical observations in treatment-seeking populations, where heavy drinkers frequently present with symptoms overlapping MDD criteria, including anhedonia, psychomotor retardation, and pervasive guilt, often persisting into early abstinence and complicating recovery trajectories (Cheng et al. 2020).

The dose-dependent nature of ethanol’s depressogenic effects is underscored by cross-national studies. For example, the World Health Organization’s (WHO) Global Burden of Disease project identified alcohol as a leading risk factor for disability-adjusted life years (DALYs) attributable to depressive disorders, with a linear relationship between per capita alcohol consumption and population-level depression prevalence. This pattern is particularly pronounced in regions with high rates of binge drinking, such as Eastern Europe and parts of Asia, where heavy episodic drinking amplifies neurochemical disruptions linked to mood dysregulation. However, the causal pathways remain complex (Organization 2009). While some individuals initiate heavy drinking to alleviate preexisting depressive symptoms—a phenomenon termed “self-medication hypothesis”—neuroadaptations caused by chronic ethanol exposure often perpetuate a cycle of worsening effects. Clinical cohorts reveal that approximately 40% of AUD patients meet the criteria for MDD during active addiction, with 15–20% continuing to exhibit treatment-resistant depressive symptoms after achieving sobriety, suggesting ethanol-induced neurotoxicity may cause irreversible alterations in mood-regulating circuits (McHugh and McBride 2020; Turner et al. 2018).

Neuroimaging and biomarker studies have provided mechanistic insights into this association. Structural MRI analyses consistently show reduced gray matter volume in the prefrontal cortex (PFC), hippocampus, and anterior cingulate cortex (ACC) of chronic drinkers, regions integral to emotional regulation and stress response. These volumetric deficits correlate with functional impairments: positron emission tomography (PET) scans reveal hypometabolism in the dorsolateral PFC and hyperactivation of the amygdala during negative emotional stimuli, mirroring neural signatures observed in non-alcoholic MDD patients (Amico et al. 2011). Furthermore, heavy drinkers exhibit elevated levels of peripheral inflammatory markers such as interleukin-6 (IL-6) and C-reactive protein (CRP), which cross the blood–brain barrier to promote neuroinflammation—a shared pathway in both ethanol-induced neurotoxicity and depression (Adams et al. 2020). These findings are complemented by neuropathological evidence from postmortem studies, where chronic alcoholics show reduced density of serotonin (5-HT) transporters in the raphe nuclei and diminished hippocampal neurogenesis, paralleling changes seen in depressed individuals without AUD (Pierson et al. 2024).

Critically, the temporal relationship between ethanol consumption and depression has been clarified by prospective cohort studies. Research following twins discordant for AUD found that the ethanol-exposed siblings developed depression at twice the rate of their non-exposed counterparts, even when controlling for shared genetic and environmental factors (Sihvola et al. 2008). Similarly, a 10-year follow-up of the Dunedin Multidisciplinary Health and Development Study revealed that adolescents who initiated heavy drinking before age 18 were 60% more likely to develop MDD by their mid-30s compared to late-onset drinkers, independent of baseline depressive symptoms. These data suggest that ethanol, particularly during neurodevelopment, may sensitize neural circuits to subsequent stress, thereby increasing vulnerability to mood disorders (Khalifeh et al. 2024). Animal models corroborate this hypothesis, as rodents exposed to ethanol during adolescence exhibit lifelong hyperreactivity of the hypothalamic–pituitary–adrenal (HPA) axis and blunted hippocampal neuroplasticity—traits that predispose to depression-like behaviors (Logrip et al. 2013; Cascino and Monteleone 2024).

Clinical observations further illuminate the interplay between ethanol and depression. Inpatient populations with severe AUD often present with “secondary depression,” characterized by melancholic features such as diurnal mood variation and non-reactive affect, which resolve partially with sustained abstinence (McHugh and Weiss 2019). However, a subset of patients—particularly those with a family history of mood disorders or polymorphisms in genes such as BDNF Val66Met or SLC6A4 (serotonin transporter)—develop persistent depressive symptoms resistant to standard antidepressants. This heterogeneity underscores the multifactorial etiology of ethanol-induced depression, where genetic susceptibility, epigenetic modifications, and environmental stressors converge to shape clinical outcomes (Bednarova et al. 2023). For example, genome-wide association studies (GWAS) have identified overlapping risk loci for AUD and MDD, including variants near DRD2 (dopamine receptor D2) and FTO (involved in oxidative stress), suggesting shared molecular pathways (Evangelia et al. 2025).

The role of polysubstance use further complicates the clinical picture. Chronic drinkers frequently co-use nicotine, cannabis, or stimulants, each modulating depression risk through distinct mechanisms. Nicotine, for instance, transiently alleviates ethanol-induced anhedonia via nicotinic acetylcholine receptor (nAChR) activation but exacerbates withdrawal-related dysphoria (Dohrman and Reiter 2003). Conversely, cannabis co-use may attenuate ethanol-related neuroinflammation through cannabinoid receptor 1 (CB1)-mediated effects but worsen cognitive deficits linked to depression. Such interactions highlight the challenges of isolating ethanol’s specific contributions to mood disorders in real-world populations (Rouzer et al. 2023).

Despite these complexities, consensus is emerging on key clinical indicators of ethanol-induced depression. Heavy drinkers with depressive symptoms typically exhibit (1) later age of MDD onset (post-25 years), (2) predominance of somatic symptoms (e.g., fatigue, insomnia) over cognitive rumination, and (3) partial responsiveness to selective serotonin reuptake inhibitors (SSRIs), which may reflect underlying serotonergic deficits compounded by ethanol’s neurotoxic effects (Pedrelli et al. 2016; Bravo et al. 2018; Al-Harbi 2012). Notably, the Hamilton Depression Rating Scale (HAMD-17) scores in this population correlate strongly with biomarkers of hepatic dysfunction (e.g., elevated gamma-glutamyl transferase) and neuronal injury (e.g., serum neurofilament light chain), suggesting a unique pathophysiology blending systemic and central nervous system (CNS) toxicity (Mirabdolhagh Hazaveh et al. 2015).

In light of these findings, the evidence linking chronic ethanol consumption to depression disorders is robust and multidimensional, spanning epidemiological, clinical, and neurobiological domains. While bidirectional causality and comorbid confounders pose interpretive challenges, longitudinal studies, and advanced neuroimaging techniques have progressively disentangled ethanol’s specific contributions to mood dysregulation. Key takeaways include the dose-dependent neurotoxicity of ethanol, the vulnerability of developing brains to persistent affective changes, and the interplay between genetic susceptibility and environmental stressors in shaping clinical outcomes. These insights not only validate preclinical models but also inform therapeutic strategies targeting shared mechanisms—such as neuroinhibition, oxidative stress, and neurotrophic deficits—to break the cycle of ethanol-induced depression. Future research must prioritize high-resolution phenotyping of AUD-MDD subtypes, multi-omics data integration, and biomarkers development to guide personalized interventions in this high-risk population.

Uncovering the Mechanisms: Insights from Animal Models

Animal models have been indispensable in unraveling the molecular and behavioral underpinnings of ethanol-induced depression, offering controlled paradigms to isolate ethanol’s neurotoxic effects from the confounding variables inherent in human studies. By simulating chronic high-dose ethanol exposure, researchers have identified conserved pathways linking ethanol consumption to depressive-like behaviors, including dysregulation of neurotrophic signaling, neurotransmitter systems, and synaptic plasticity. These models range from voluntary oral self-administration in rodents to forced vapor inhalation protocols, each capturing distinct facets of human alcoholism. For instance, chronic intermittent ethanol (CIE) exposure—a widely used model mimicking binge-drinking patterns—induces persistent anhedonia and despair-like behaviors in mice, as evidenced by reduced sucrose preference in the two-bottle choice test and increased immobility in the forced swim test (FST). These behavioral changes parallel clinical observations of anhedonia and psychomotor retardation in heavy drinkers, validating the translational relevance of such paradigms (Xiao et al. 2023).

Central to these models is replicating ethanol’s dose-dependent effects on mood. Rats exposed to ethanol vapor for 12–16 weeks exhibit progressive escalation of depressive-like behaviors, correlating with blood ethanol concentrations (BECs) exceeding 200 mg/dL, a threshold associated with severe intoxication in humans. Neurochemically, these animals display reduced hippocampal brain-derived neurotrophic factor (BDNF) expression, mirroring the 50% BDNF deficits observed in postmortem brains of chronic alcoholics (Yao et al. 2021). BDNF knockdown experiments further cement its role: mice with conditional BDNF deletion in the ventral tegmental area (VTA) or hippocampus develop spontaneous depressive phenotypes, even in the absence of ethanol, while BDNF infusion into these regions rescues ethanol-induced anhedonia (Zhong et al. 2018). These findings align with human PET studies showing diminished BDNF-CREB (cAMP response element-binding protein) signaling in depressed alcoholics, underscoring the conserved nature of neurotrophic disruptions across species.

The serotonin (5-HT) system, a key mediator of mood regulation, has been extensively probed in animal models. Chronic ethanol exposure reduces tryptophan hydroxylase (TPH2) activity—the rate-limiting enzyme in 5-HT synthesis—in the dorsal raphe nuclei (DRN) of rodents, leading to depleted 5-HT levels in the prefrontal cortex (PFC) and amygdala. Pharmacological interventions that restore 5-HT signaling, such as selective serotonin reuptake inhibitors (SSRIs), ameliorate depressive-like behaviors in ethanol-exposed mice, but only in subsets of animals, echoing the variable efficacy of SSRIs in human AUD populations (Khan et al. 2023). This heterogeneity is partly genetic: mouse strains with polymorphisms in the Slc6a4 gene (encoding the 5-HT transporter) exhibit divergent responses to ethanol, with short-allele carriers showing heightened vulnerability to depression-like phenotypes. Such strain differences mirror human GWAS data linking SLC6A4 variants to AUD-MDD comorbidity, highlighting the utility of animal models in dissecting gene-environment interactions (Carneiro et al. 2009).

Dopamine (DA) dysregulation, another hallmark of ethanol-induced depression, has been elucidated through optogenetic and chemogenetic techniques. Chronic ethanol consumption blunts DA release in the nucleus accumbens (NAc) during reward anticipation, a deficit linked to reduced firing of VTA DA neurons. Remarkably, optogenetic stimulation of VTA-NAc projections restores motivation and sucrose preference in ethanol-exposed mice, suggesting DAergic hypoactivity drives anhedonia. These insights dovetail with human fMRI studies showing attenuated ventral striatal activation during reward tasks in depressed alcoholics. Ethanol’s impact on GABA and glutamate systems further compounds affective dysfunction. Prolonged ethanol intake upregulates GluN2B-containing NMDA receptors in the hippocampus, enhancing excitotoxicity and neuronal apoptosis, while downregulating δ-subunit-containing GABAA receptors in the amygdala, disinhibiting fear circuits. Pharmacological blockade of GluN2B or allosteric modulation of GABAA receptors normalizes anxiety and despair behaviors in ethanol-fed rodents, pinpointing these receptors as therapeutic targets (Zhang et al. 2024; Carroll et al. 2006).

Oxidative stress and neuroinflammation, mechanistically intertwined with ethanol’s neurotoxicity, have been characterized through biomarker and histopathological analyses. Chronic ethanol administration in rats elevates reactive oxygen species (ROS) in the PFC and hippocampus, depleting glutathione (GSH) and impairing mitochondrial complex I activity. These oxidative insults activate microglia and astrocytes, triggering pro-inflammatory cytokine release (e.g., IL-1β, TNF-α) and propagating neuronal damage (Teixeira et al. 2014; Fernández-Rodríguez et al. 2022). Interventions targeting oxidative stress—such as N-acetylcysteine (NAC) or coenzyme Q10—reduce depressive-like behaviors and restore redox balance in ethanol-exposed animals (Kim et al. 2024). Similarly, minocycline, a microglial inhibitor, attenuates ethanol-induced neuroinflammation and anhedonia, supporting the role of glial activation in mood dysregulation. Postmortem analyses of ethanol-fed mice reveal increased caspase-3 activity and TUNEL-positive cells in the hippocampus, confirming apoptosis as a key mechanism of ethanol-induced neuronal loss (Ren et al. 2019).

Epigenetic modifications, particularly histone deacetylase (HDAC)-mediated chromatin remodeling, have emerged as a critical interface between chronic ethanol exposure and depressive phenotypes. Mouse models demonstrate that ethanol upregulates HDAC2/3 in the NAc and PFC, repressing genes involved in synaptic plasticity (e.g., Arc, Bdnf). Pharmacological HDAC inhibitors, such as sodium butyrate or CI-994, reverse these transcriptional changes and rescue ethanol-induced social withdrawal and despair behaviors. These findings informed human PET studies using the HDAC tracer [11C] Martinostat, which confirmed elevated HDAC expression in the brains of chronic drinkers. Similarly, DNA methyltransferase (DNMT) inhibitors alleviate depressive-like behaviors in ethanol-exposed rodents by demethylating BDNF promoters, underscoring the therapeutic potential of epigenetic editing (Shivakumar et al. 2020).

Stress-axis dysregulation, a shared feature of depression and alcoholism, has been modeled through chronic mild stress (CMS) paradigms. Ethanol-exposed rats subjected to CMS exhibit exaggerated corticosterone responses and glucocorticoid receptor (GR) resistance in the PFC and hippocampus, mimicking the HPA axis hyperactivity observed in depressed humans (Alhaddad et al. 2020). CRF (corticotropin-releasing factor) antagonists, which block stress signaling, normalize both HPA axis function and depressive behaviors in these models, suggesting CRF hyperactivation mediates ethanol’s stress-sensitizing effects (Huang et al. 2010).

Despite their utility, animal models face limitations in replicating the psychosocial dimensions of human depression, such as rumination or existential hopelessness. Moreover, species differences in ethanol metabolism (e.g., higher catalase activity in rodents) and receptor subtype distribution (e.g., GABAA α-subunits) necessitate cautious extrapolation to humans. Nevertheless, the conserved molecular pathways identified in these models—from BDNF depletion to HDAC-mediated gene silencing—provide a mechanistic scaffold for understanding ethanol-induced depression. Emerging techniques like single-cell RNA sequencing and in vivo calcium imaging promise to refine these models further, enabling real-time tracking of ethanol’s effects on specific neuronal populations. By bridging molecular insights with behavioral outcomes, animal research continues to illuminate the dark nexus between chronic ethanol consumption and depression, guiding the development of targeted therapies for this devastating comorbidity.

Translating Findings to Humans: Clinical Studies on Chronic Ethanol Consumption and Depression

The translation of preclinical findings from animal models to human clinical studies represents a critical bridge in understanding the molecular underpinnings of ethanol-induced depression. While animal studies have provided foundational insights into the neurobiological consequences of chronic ethanol exposure, human research faces unique challenges stemming from the heterogeneity of human populations, ethical limitations, and the multifactorial nature of alcoholism and mood disorders (Vena et al. 2020). Nevertheless, advances in neuroimaging, biomarker analysis, and longitudinal cohort studies have enabled researchers to corroborate key mechanistic pathways in humans, particularly those involving neurotrophic deficits, hippocampal atrophy, and neurotransmitter dysregulation. This section examines the successes and limitations of translating preclinical evidence to clinical research, with a focus on how chronic ethanol consumption disrupts mood and cognitive function in human populations.

One of the most significant challenges in human studies is disentangling the bidirectional relationship between chronic ethanol use and depression. Epidemiological data consistently reveal that individuals with AUD exhibit a two- to threefold higher risk of developing MDD compared to the general population. However, clinical heterogeneity—such as variations in drinking patterns, genetic susceptibility, and comorbid psychiatric conditions—complicates causal inferences. For instance, heavy drinkers often report self-medication for preexisting depressive symptoms, while others develop mood disturbances as a direct consequence of neurotoxic ethanol effects. Longitudinal studies, such as the NESARC, have attempted to address this complexity by tracking temporal relationships. These studies suggest that chronic ethanol consumption (> 80 g/day for ≥ 5 years) independently predicts subsequent depressive episodes, even after adjusting for baseline mental health status. Such findings align with preclinical models showing that prolonged ethanol exposure induces persistent neuroadaptations in mood-regulating circuits, including the prefrontal cortex, hippocampus, and amygdala (Hasin and Grant 2015).

Neuroimaging has been instrumental in bridging animal and human research. PET studies using tracers like [11C] Martinostat have quantified HDAC activity in chronic drinkers, revealing epigenetic modifications that suppress neurotrophic signaling (Dagnew et al. 2024). For example, reduced hippocampal BDNF expression—a hallmark of ethanol’s neurotoxicity in rodents—has been corroborated in humans, with chronic drinkers showing 50% lower BDNF levels compared to controls. These deficits correlate with structural MRI findings of 30% gray matter loss in hippocampal subfields, particularly the CA1 and dentate gyrus, regions critical for memory consolidation and emotional regulation. Importantly, such volumetric reductions are not merely artifacts of neurodegeneration; they are mechanistically tied to functional impairments (Logrip et al. 2009). Patients with hippocampal atrophy perform poorly on cognitive tests like the Rey Auditory Verbal Learning Test (RAVLT), which assesses verbal memory, and exhibit elevated scores on the HAMD-17, a standardized measure of depressive symptom severity. The strength of these correlations (r = 0.62 for RAVLT; r = − 0.71 for HAMD-17) underscores the clinical relevance of preclinical findings linking BDNF depletion to both cognitive decline and affective dysfunction (Putcha et al. 2019; Yao et al. 2020).

Despite these advances, translating interventions from animal models to humans remains fraught with difficulty. Rodent studies often employ controlled ethanol administration regimens, whereas human drinking behavior is influenced by sociocultural factors, polysubstance use, and fluctuating motivation. Moreover, many preclinical strategies—such as HDAC inhibitors or BDNF infusions—face pharmacokinetic and safety barriers in clinical trials. For example, while HDAC inhibitors reverse ethanol-induced synaptic deficits in mice, their systemic administration in humans risks off-target effects, including gastrointestinal distress and immunosuppression (Arora et al. 2013). Similarly, attempts to boost BDNF levels via exercise or pharmacotherapy have yielded mixed results in AUD populations, partly due to individual differences in baseline neurotrophic reserves and genetic polymorphisms. The BDNF Val66Met variant, present in 30% of humans, attenuates activity-dependent BDNF secretion and has been linked to poorer outcomes in alcoholics with comorbid depression. Such genetic variability highlights the need for personalized approaches in translating mechanistic insights (Wojnar et al. 2009).

Cognitive and affective assessments further illustrate the translational gap. While rodents exhibit depression-like behaviors (e.g., anhedonia in sucrose preference tests) after chronic ethanol exposure, human depression is a multifaceted construct encompassing emotional, cognitive, and somatic symptoms (Olney et al. 2018). Clinical tools like the HAMD-17 capture this complexity but lack the granularity to isolate specific neurobiological pathways. Neuropsychological batteries, including the RAVLT and Trail Making Test, provide objective measures of ethanol’s cognitive toll, yet they often fail to distinguish between deficits caused by direct neurotoxicity and those secondary to comorbid conditions like hepatic encephalopathy or thiamine deficiency. This limitation underscores the importance of integrating biomarkers into clinical research (Brust 2010). Recent studies have combined neuroimaging with peripheral measures of oxidative stress (e.g., lipid peroxidation markers) and neuroinflammation (e.g., pro-inflammatory cytokines) to establish mechanistic links between ethanol’s molecular effects and behavioral outcomes. For instance, elevated plasma levels of IL-6 and CRP in heavy drinkers correlate with both hippocampal atrophy and depressive symptom severity, mirroring rodent data on ethanol-induced glial activation and blood–brain barrier disruption (Adams et al. 2020; Volpato et al. 2004).

The role of neurotransmitter systems in ethanol-induced depression offers another translational success story. Preclinical models have long implicated 5-HT and DA dysregulation in ethanol-related affective disturbances. Human PET studies using radioligands like [11C] DASB (a 5-HT transporter tracer) and [11C] raclopride (a DA D2/3 receptor antagonist) have validated these findings, showing reduced 5-HT transporter density in the raphe nuclei and blunted DA release in the striatum of chronic drinkers. These deficits, which persist into early abstinence, align with clinical observations of anhedonia and emotional blunting in AUD patients (Savitz and Drevets 2013). Furthermore, pharmacotherapeutic trials targeting monoaminergic systems—such as SSRIs and bupropion—have shown modest efficacy in reducing depressive symptoms in alcoholics, though their benefits are often overshadowed by high placebo response rates and relapse risk (Agabio et al. 2018). This discrepancy highlights a critical translational lesson: while animal models excel at isolating specific pathways, human depression arises from interconnected networks that demand multimodal treatment strategies.

Emerging research on synaptic plasticity and epigenetics has further narrowed the translational divide. Rodent studies demonstrate that chronic ethanol impairs long-term potentiation (LTP) in the hippocampus, a process essential for learning and memory (Hedayati-Moghadam et al. 2024). Human electrophysiological studies, utilizing transcranial magnetic stimulation (TMS) paired with electroencephalography (EEG), have detected analogous deficits in cortical plasticity, with heavy drinkers showing attenuated LTP-like responses in the motor cortex (Avchalumov and Mandyam 2020). These functional impairments are compounded by epigenetic changes, such as hypermethylation of BDNF promoters and histone modifications at glutamate receptor genes, which are detectable in the postmortem brains of alcoholics. Notably, interventions that restore synaptic plasticities—such as NMDA receptor modulators or transcranial direct current stimulation (tDCS)—are now being explored in clinical trials, offering hope for mechanistically grounded therapies (Taqi et al. 2011).

Taken together, while challenges persist in translating preclinical findings to human studies, the integration of advanced neuroimaging, genetic profiling, and biomarker analysis has significantly advanced our understanding of ethanol-induced depression. Key successes include the validation of BDNF deficits, hippocampal atrophy, and monoaminergic dysfunction as central mechanisms in human populations. However, the field must confront the limitations of reductionist models by embracing systems-level approaches that account for the biological and environmental complexity of AUD and depression. Future research should prioritize longitudinal designs, multimodal biomarker panels, and targeted interventions that address both molecular and psychosocial contributors to this debilitating comorbidity. By bridging the translational gap, such efforts will not only elucidate the pathophysiology of ethanol-induced depression but also pave the way for therapies that restore neural resilience and improve clinical outcomes.

Molecular Mechanisms

Oxidative Stress Pathways in Alcohol-Induced Neural Damage

Chronic ethanol consumption has been recognized as a significant contributor to various neuropsychiatric disorders, among which depression stands out as a prevalent and debilitating condition. The relationship between ethanol-induced depression and oxidative stress has garnered considerable attention in recent years, revealing a complex interplay of biochemical processes that underlie the pathophysiology of depressive disorders. Oxidative stress refers to an imbalance between the production of reactive oxygen species (ROS) and the body’s ability to detoxify these harmful byproducts or repair the resulting damage (Baghcheghi et al. 2016). This section explores how chronic ethanol consumption leads to increased oxidative stress and the subsequent molecular mechanisms that contribute to the development of depression.

When ethanol is metabolized in the liver, it undergoes a series of enzymatic reactions primarily mediated by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). These processes convert ethanol to acetaldehyde and acetic acid (Ajoolabady et al. 2022; Aslam and Kwo 2023; Shin et al. 2021). While ethanol metabolism is necessary for its elimination from the body, it also generates ROS as byproducts. Elevated levels of ROS can lead to oxidative damage to cellular components, including lipids, proteins, and DNA. This oxidative damage disrupts cellular functions and contributes to neurodegenerative processes that are often associated with depression (Gorlova et al. 2023).

One of the key players in the oxidative stress response is the mitochondrion. As the primary site of energy production through oxidative phosphorylation, mitochondria are also a significant source of ROS. Chronic ethanol consumption can lead to mitochondrial dysfunction, which increases mitochondrial permeability and causes the release of pro-apoptotic factors. This dysfunctional state not only increases ROS production but also weakens the cell’s capacity to manage oxidative stress (Prasun et al. 2021). Mitochondrial dysfunction has been implicated in various neurodegenerative diseases, and its role in ethanol-induced depression is becoming increasingly recognized (Bustamante-Barrientos et al. 2023).

The brain is particularly vulnerable to oxidative stress due to its high oxygen consumption, lipid-rich environment, and limited antioxidant defenses. Neurons contain relatively low levels of antioxidant enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase compared to other cell types (Wang et al. 2021). As a result, when ROS levels rise due to chronic ethanol exposure, neurons are at a heightened risk for oxidative damage. This damage can manifest as lipid peroxidation, which alters membrane integrity and fluidity, leading to cell death and neuroinflammation—both of which are implicated in the development of depressive symptoms (Mashayekhi-sardoo et al. 2025).

In addition to direct oxidative damage, chronic ethanol consumption can lead to increased levels of pro-inflammatory cytokines, which are known to exacerbate oxidative stress. Ethanol metabolism can activate the nuclear factor kappa B (NF-κB) signaling pathway, leading to the transcription of genes encoding pro-inflammatory cytokines such as TNF-α, interleukin-1 beta (IL-1β), and IL-6 (Yang et al. 2022). These inflammatory mediators can further promote oxidative stress by inducing the production of ROS and inhibiting the expression of antioxidant enzymes. The resulting neuroinflammatory environment can contribute to the neurodegenerative processes associated with depression.

Another critical mechanism by which oxidative stress contributes to ethanol-induced depression involves the dysregulation of neurotransmitter systems. Chronic oxidative stress can impair the synthesis and function of key neurotransmitters, particularly serotonin and dopamine, essential for mood regulation. For example, oxidative damage to tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, can decrease serotonin levels in the brain (Correia and Vale 2024; Qiu et al. 2015). Similarly, oxidative stress can affect dopamine synthesis and receptor signaling, further exacerbating mood dysregulation (Correia and Vale 2024). The resulting neurotransmitter imbalances can contribute significantly to the onset and maintenance of depressive symptoms in individuals with chronic ethanol exposure.

Moreover, oxidative stress has been linked to disruptions in the HPA axis, which plays a crucial role in the body’s response to stress. Chronic ethanol consumption can lead to hyperactivation of the HPA axis, resulting in elevated cortisol levels (Dunne and Ivers 2023; Rachdaoui and Sarkar 2017). Cortisol, a glucocorticoid hormone, is known to have neurotoxic effects when present in excess. It can induce apoptosis in neurons and further promote oxidative stress through the activation of reactive microglia (Motta et al. 2021; Juszczyk et al. 2021). This cascade of events can create a vicious cycle, where increased oxidative stress leads to heightened HPA axis activity, which in turn perpetuates oxidative damage and mood dysregulation.

Additionally, chronic ethanol exposure can disrupt the balance of antioxidant defenses in the brain. The production of antioxidants, such as glutathione, is often impaired in individuals with a history of heavy alcohol use. Glutathione is a crucial tripeptide that acts as a primary antioxidant, helping to neutralize ROS and maintain redox balance within cells. Ethanol-induced oxidative stress can deplete glutathione levels, reducing the brain’s capacity to counteract oxidative damage (Tsermpini et al. 2022). This depletion can further exacerbate the vulnerability of neurons to oxidative stress, contributing to the onset of depressive symptoms.

The relationship between oxidative stress and neurodegeneration is also closely tied to the activation of apoptotic pathways (Emerit et al. 2004; Radi et al. 2014). Chronic ethanol consumption can induce oxidative stress-mediated apoptosis in neurons through the activation of caspases, a family of proteases that play essential roles in the programmed cell death process (Chen et al. 2008; Ku et al. 2007). For instance, caspase-3 is often activated in response to oxidative stress and is responsible for executing apoptosis by cleaving various cellular substrates (Han et al. 2005; Young et al. 2005). The loss of neuronal survival through apoptosis can lead to cognitive deficits, emotional disturbances, and ultimately, the manifestation of depressive disorders.

Furthermore, the impact of oxidative stress on circadian rhythms and sleep patterns cannot be overlooked in the context of ethanol-induced depression. Sleep disruptions are commonly reported in individuals with depression and are often exacerbated by chronic alcohol consumption (Firth et al. 2020; He et al. 2019). Oxidative stress has been shown to influence sleep regulation through its effects on neurotransmitter systems, particularly those involving GABA and serotonin (Tsermpini et al. 2022; Sahin et al. 2024). Poor sleep quality and disturbances in circadian rhythms can further contribute to the development and persistence of depressive symptoms.

In addition to these mechanisms, it is important to consider the potential for oxidative stress to affect neuroplasticity and cognitive function. Chronic ethanol exposure can impair synaptic plasticity, the ability of synapses to strengthen or weaken over time, which is crucial for learning and memory. Oxidative stress has been shown to negatively impact LTP and long-term depression (LTD), both of which are essential for synaptic adaptation (Li et al. 2024; Kazemi et al. 2024). Impaired neuroplasticity can lead to cognitive deficits and emotional dysregulation, further contributing to the symptomatology of depression.

While the understanding of oxidative stress in chronic ethanol consumption and its role in depression is rapidly evolving, it is important to acknowledge the individual variability in response to alcohol. Genetic predispositions, environmental factors, and prior history of trauma or mental health disorders can influence how individuals experience oxidative stress and its effects on mood (Hawn et al. 2020; Zhan et al. 2022; Al-Kufaishi and Al-Musawi 2024; Ajoolabady et al. 2022; Borruto et al. 2021; Hilal et al. 2024). Additionally, the timing, quantity, and duration of alcohol consumption can all play a significant role in determining the extent of oxidative damage and its subsequent impact on mental health. Table 1 summarizes the effects of ethanol on the antioxidant enzymes and oxidative status in different parts of the brain.

Table 1.

Effects of ethanol on the antioxidant enzymes and the oxidative status in different parts of the brain

Tissue	TBARS	LPO	COX	SOD	GPx	CAT	GSH	Ref
Hippocampus	↑			↓	↓			(Taati et al. 2011)
Hippocampus				↓		↓	↓	(Phunchago et al. 2015)
Hippocampus				↓	↓	↓		(Soleimani et al. 2016)
Hippocampus	↑			↔	↔			(Gonenc et al. 2005)
Hippocampus				↑		↓		(Baradaran et al. 2021)
Hippocampus			↑	↓		↓	↓	(Song et al. 2015)
Cortex				↓		↓		(Baradaran et al. 2021)
Hippocampus and Cortex		↑					↓	(Baydas et al. 2005)
Cerebral Cortex				↕		↑		(Heaton et al. 2003b)
Striatum				↓		↓	↓	(Heaton et al. 2003a)
Forebrain				↓			↓	(Raghavendra and Kulkarni 2001)

GPx Glutathione peroxidase, SOD Superoxide dismutase, TBARS Thiobarbituric acid reactive substances, LPO Lipid peroxidation, CAT Catalase, GSH Glutathione

Neuroinflammatory Responses to Chronic Alcohol Exposure

Chronic ethanol consumption is increasingly recognized as a significant risk factor for developing various neuropsychiatric disorders, with depression being one of the most prevalent outcomes. One of the pivotal mechanisms through which chronic alcohol use leads to depressive symptoms is neuroinflammation, a complex biological response characterized by the activation of glial cells, the release of pro-inflammatory cytokines, and the disruption of neuronal function. Understanding the intricate relationship between ethanol-induced neuroinflammation and depression is essential for unraveling the pathophysiological processes underlying these conditions. This section delves into the molecular mechanisms by which chronic ethanol consumption results in depression through the promotion of neuroinflammation.

Neuroinflammation is initiated primarily by the activation of microglia, the resident immune cells of the central nervous system (Fornari Laurindo et al. 2024). Under normal physiological conditions, microglia play a crucial role in maintaining homeostasis, supporting neuronal health, and responding to injury. However, chronic ethanol exposure leads to the dysregulation of microglial function, resulting in a persistent state of inflammation. Ethanol induces the production of pro-inflammatory mediators, such as cytokines and chemokines, which can alter neuronal signaling, disrupt synaptic function, and ultimately contribute to mood dysregulation (Mikhalitskaya et al. 2024).

Upon chronic exposure to ethanol, microglia become activated through several pathways. One critical pathway involves the NF-κB signaling pathway. Ethanol metabolism can lead to the production of ROS and various metabolites that activate NF-κB, a transcription factor that regulates the expression of numerous pro-inflammatory genes. Once activated, NF-κB translocates to the nucleus, initiating the transcription of cytokines such as TNF-α, IL-1β, and IL-6 (Mikhalitskaya et al. 2024). These pro-inflammatory cytokines create a neuroinflammatory environment that can alter neurotransmitter signaling and contribute to the development of depressive symptoms.

Additionally, ethanol can activate the inflammasome, a multi-protein complex that plays a key role in the innate immune response. The NLRP3 (NOD-like receptor family pyrin domain containing 3) inflammasome is particularly relevant in the context of ethanol-induced neuroinflammation (De Filippis et al. 2016; Hoyt et al. 2017). Chronic ethanol exposure has been shown to promote NLRP3 inflammasome activation, leading to the maturation and release of IL-1β and IL-18, two potent pro-inflammatory cytokines (Alfonso-Loeches et al. 2014). The release of these cytokines not only exacerbates neuroinflammation but also impacts synaptic plasticity and neuronal survival, leading to cognitive deficits and emotional disturbances commonly associated with depression (Liu et al. 2022).

The effects of neuroinflammation on neurotransmitter systems are particularly noteworthy. Chronic inflammation can disrupt the synthesis, release, and reuptake of key neurotransmitters, including serotonin, dopamine, and norepinephrine. For instance, increased levels of TNF-α and IL-6 can inhibit the activity of tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, resulting in reduced serotonin levels (Qiu et al. 2015). Similarly, pro-inflammatory cytokines can impair dopamine signaling by affecting the expression of dopamine receptors and transporters (Kaur et al. 2017). These alterations in neurotransmitter systems are critical in the development of depressive symptoms, as they directly influence mood regulation and emotional well-being.

Another crucial aspect of neuroinflammation related to chronic ethanol consumption is its impact on the blood–brain barrier (BBB). The BBB is a highly selective permeability barrier that protects the brain from harmful substances and maintains homeostasis. Chronic ethanol exposure can lead to BBB disruption through the activation of microglia and the release of pro-inflammatory mediators. Elevated levels of cytokines such as TNF-α can increase the expression of adhesion molecules and tight junction proteins, compromising the integrity of the BBB (Vore and Deak 2021). This disruption allows peripheral inflammatory cytokines and other potentially neurotoxic substances to infiltrate the CNS, further exacerbating neuroinflammation and contributing to the development of depressive symptoms.

In addition to the direct effects of neuroinflammation on neuronal health, chronic alcohol exposure can also lead to the activation of astrocytes, another type of glial cell. Astrocytes play a crucial role in maintaining the extracellular environment, supporting neuronal function, and regulating neurotransmitter levels. However, chronic ethanol consumption can lead to astrogliosis, a condition characterized by the proliferation and activation of astrocytes. Activated astrocytes release a variety of pro-inflammatory cytokines and reactive gliosis factors that can further contribute to neuroinflammation (Kane and Drew 2021). The dysregulation of astrocytic function can also impair the uptake of neurotransmitters such as glutamate, leading to excitotoxicity and neuronal damage (Kamal et al. 2020). This cascade of events can significantly impact mood regulation and cognitive function, further reinforcing the link between neuroinflammation and depression.

Neuroinflammation is also associated with alterations in neurotrophic factors, such as brain-derived neurotrophic factor (BDNF). BDNF is crucial for neuronal survival, growth, and synaptic plasticity (Baghcheghi et al. 2021). Chronic inflammation can lead to decreased BDNF levels, which is often observed in individuals with depression. The reduction of BDNF compromises neuroplasticity and the brain’s ability to adapt to stressors, contributing to the pathophysiology of depression (Correia and Vale 2024; Motta et al. 2021). Ethanol-induced neuroinflammation can disrupt the signaling pathways that promote BDNF expression, leading to a vicious cycle where reduced neurotrophic support exacerbates mood dysregulation and cognitive impairments (Hauser et al. 2011; Peregud et al. 2023).

Moreover, the relationship between neuroinflammation and oxidative stress is intertwined in the context of chronic ethanol consumption. Ethanol induces oxidative stress through various mechanisms, including mitochondrial dysfunction and the generation of ROS during its metabolism (Alfonso-Loeches et al. 2014). Oxidative stress, in turn, can activate inflammatory pathways, creating a feedback loop that perpetuates both oxidative damage and neuroinflammation. This synergy between oxidative stress and neuroinflammation is critical in understanding the molecular mechanisms underlying ethanol-induced depression. The combined effects of increased ROS production and inflammatory cytokine release can further impair neuronal function and viability, leading to the manifestation of depressive symptoms.

Chronic ethanol consumption can also influence the gut-brain axis, which plays a significant role in modulating neuroinflammation and mood regulation. The gut microbiome is involved in the production of various metabolites that can affect brain health. Alcohol can alter gut microbiota composition, leading to dysbiosis and increased intestinal permeability, which allows the translocation of bacterial endotoxins into the bloodstream. These endotoxins can activate systemic inflammation, further exacerbating neuroinflammation in the CNS (Bishehsari et al. 2017). The interplay between gut dysbiosis, systemic inflammation, and neuroinflammation highlights the complexity of the mechanisms by which chronic ethanol consumption can contribute to depressive disorders.

While the mechanisms discussed above illustrate the significant impact of neuroinflammation on ethanol-induced depression, it is essential to recognize the individual variability in response to alcohol consumption. Genetic predispositions, prior mental health conditions, and environmental stressors can all influence how neuroinflammation manifests and contributes to depressive symptoms (Bishehsari et al. 2017). Understanding these individual differences is crucial for developing targeted interventions aimed at mitigating the effects of neuroinflammation in those affected by chronic alcohol use. Table 2 presents a summary of the ethanol effect on inflammatory factors and its effects on specific brain regions.

Table 2.

The ethanol effect on inflammatory factors and its effects on specific brain regions

Tissue	TNF-α	TGF-β1	NF-kB	COX2	SENP6	IL-6	IL-1	iNOS	TLR4	MCP1	Ref
Hippocampus	↓					↑				↑	(Kane et al. 2014)
Hippocampal	↑										(Baradaran et al. 2021)
Hippocampal							↓				(Song et al. 2015)
Cerebellum						↑				↑	(Kane et al. 2014)
Cerebral cortex										↑	(Kane et al. 2014)
Hippocampus and cortex			↓	↑							(Crews et al. 2006)
Hippocampus and cortex	↑	↑	↑				↑				(Tiwari and Chopra 2012)
Activated Astroglial cells				↑				↑	↑		(Blanco et al. 2005)
Ethanol-induced Glial activation	↑					↑	↑		↑		(Alfonso-Loeches et al. 2010)
Activated Microglial cells	↑		↑		↑	↑					(Li et al. 2019)

TNF-α Tumor necrosis factor-alpha, NF-kB Nuclear factor-kB, COX2 Cyclooxygenase 2, IL-6 Interleukin-6, IL-1 Interleukin-1, iNOS Inducible nitric oxide synthase, TLR Toll-like receptor 4, TGF-β1 Transforming growth factor β1, SENP SUMO- specific proteases, MCP-1 Monocyte chmoattractant protein-1, ↓: Decrease; ↑: Increase

Neurotrophic Dysregulation: Impact on Neuronal Health and Function

Chronic ethanol consumption is a significant contributor to the development of various neuropsychiatric disorders, with depression being one of the most prominent outcomes. A growing body of evidence suggests that ethanol’s impact on neurotrophic factors plays a crucial role in the pathophysiology of ethanol-induced depression. Neurotrophic factors, essential proteins that support the growth, survival, and differentiation of neurons, are vital for maintaining neuronal health and function. In this section, we will explore the molecular mechanisms through which chronic ethanol consumption leads to a decrease in neurotrophic factors, particularly BDNF, and how this depletion contributes to the onset of depressive symptoms.

BDNF is a key neurotrophic factor that plays a central role in neuronal survival, growth, and synaptic plasticity. It is crucial for neurogenesis and has been implicated in mood regulation and cognitive function. In healthy individuals, BDNF promotes the survival and differentiation of neurons, facilitates synaptic plasticity, and enhances learning and memory processes. However, chronic alcohol exposure has been shown to significantly reduce BDNF levels in various brain regions, including the hippocampus and prefrontal cortex, which are critical for mood regulation. The reduction of BDNF is particularly concerning, as it is often correlated with the severity of depressive symptoms (Hauser et al. 2011; Pourfridoni et al. 2024).

The molecular mechanisms by which chronic ethanol consumption leads to decreased BDNF levels are multifaceted. One key mechanism involves ethanol-induced oxidative stress. Chronic alcohol consumption generates ROS that can damage cellular components and disrupt signaling pathways. ROS can lead to the activation of various stress-responsive signaling pathways, including the c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK) pathways (Falcicchia et al. 2020; Ku et al. 2007; Réus et al. 2016). These pathways can inhibit BDNF transcription by phosphorylating and inactivating transcription factors that are essential for BDNF gene expression. For instance, the activation of JNK can lead to the phosphorylation of cAMP response element-binding protein (CREB), a critical transcription factor that promotes BDNF expression. When CREB is phosphorylated by JNK, its ability to activate the BDNF promoter is diminished, resulting in decreased BDNF production (Motaghinejad et al. 2021; Legaki et al. 2024).

In addition to oxidative stress, neuroinflammation plays a significant role in the reduction of neurotrophic factors following chronic ethanol exposure. Ethanol consumption activates microglia, the resident immune cells in the brain, leading to a state of chronic neuroinflammation. Activated microglia release pro-inflammatory cytokines, such as TNF-α and IL-6, which can further inhibit BDNF expression (Şahin et al. 2015). For example, TNF-α has been shown to interfere with the signaling pathways that regulate BDNF transcription, including the extracellular signal-regulated kinase (ERK) pathway (De Lamirande and Gagnon 2002). The ERK pathway is essential for the activation of CREB and subsequent BDNF transcription. When pro-inflammatory cytokines like TNF-α are elevated, they can inhibit the activation of the ERK pathway, leading to reduced BDNF levels and impaired neuroplasticity (De Lamirande and Gagnon 2002).

Moreover, chronic ethanol exposure can lead to disruptions in the epigenetic regulation of BDNF expression. Epigenetic modifications, such as DNA methylation and histone acetylation, play a crucial role in regulating gene expression, including that of neurotrophic factors. Studies have shown that chronic alcohol consumption can induce hypermethylation of the BDNF gene promoter, resulting in decreased BDNF expression (Heberlein et al. 2015). This epigenetic alteration can persist long after alcohol consumption has ceased, leading to long-term deficits in BDNF levels and associated depressive symptoms. The role of epigenetic modifications underscores the complexity of how chronic ethanol exposure can result in sustained changes in neurotrophic factor levels and function.

Another significant factor contributing to the decline of neurotrophic factors in the context of chronic ethanol consumption is the impact on mitochondrial function. Mitochondria are essential for cellular energy production and play a crucial role in regulating apoptosis and oxidative stress. Chronic alcohol exposure can lead to mitochondrial dysfunction, characterized by impaired ATP production and increased ROS generation (Hoyt et al. 2017). Mitochondrial dysfunction can disrupt the signaling pathways necessary for BDNF expression. For example, decreased mitochondrial activity can lead to reduced levels of ATP, which is required for the phosphorylation of various signaling molecules involved in BDNF transcription (Motaghinejad et al. 2021). The interplay between mitochondrial dysfunction and decreased neurotrophic factor levels highlights the importance of cellular energy status in maintaining neuronal health.

The relationship between chronic ethanol exposure, neurotrophic factors, and depression is further complicated by the role of stress and the HPA axis. Chronic alcohol consumption can lead to dysregulation of the HPA axis, resulting in elevated levels of cortisol, a stress hormone known to have neurotoxic effects (Al-Kufaishi and Al-Musawi 2024; Finn 2023; Mbiydzenyuy and Qulu 2024). Elevated cortisol levels can downregulate BDNF expression through various mechanisms, including the inhibition of CREB activity and increased expression of inhibitory factors (Naveen et al. 2016). The effects of chronic stress on BDNF levels are well-documented, with stress-induced reductions in BDNF being associated with the onset of depressive symptoms. Thus, the interaction between alcohol-induced stress, HPA axis dysregulation, and neurotrophic factors is critical in understanding the development of depression in individuals with chronic alcohol use.

The decrease in neurotrophic factors such as BDNF has profound implications for neuronal health and function. Reduced BDNF levels impair synaptic plasticity, leading to deficits in learning and memory, which are often observed in individuals with depression (Peregud et al. 2023). Furthermore, the loss of neurotrophic support can lead to neurodegeneration and apoptosis, contributing to the cognitive and emotional deficits associated with ethanol-induced depression (Li et al. 2024). The interplay between reduced neurotrophic factors and neuronal health highlights the importance of neurotrophic support in maintaining mood regulation and cognitive function.

In the context of chronic ethanol exposure, the reduction of other neurotrophic factors, such as nerve growth factor (NGF) and neurotrophin-3 (NT-3), may also contribute to the pathophysiology of depression. NGF and NT-3 are involved in neuronal survival and differentiation, and their depletion can exacerbate the neurotoxic effects of ethanol. Similar to BDNF, both NGF and NT-3 are subject to regulation by the same inflammatory and oxidative stress pathways that affect BDNF levels (Malewska-Kasprzak et al. 2024). Therefore, the comprehensive impact of chronic ethanol consumption on multiple neurotrophic factors further supports the contribution of these proteins to the development of depressive disorders.

Additionally, the interaction between neurotrophic factors and the gut-brain axis is becoming increasingly relevant in the context of chronic alcohol consumption and depression. Ethanol can disrupt gut microbiota composition, leading to dysbiosis and increased intestinal permeability. This dysbiosis can produce metabolites that influence neurotrophic factor signaling pathways in the brain (Maqsood and Stone 2016). For instance, short-chain fatty acids (SCFAs) produced by gut bacteria have been shown to have neuroprotective effects and promote BDNF expression (Church et al. 2023). Chronic alcohol use, by disrupting gut health, may lead to a reduction in SCFA production, further diminishing neurotrophic support and contributing to depressive symptoms (Shen et al. 2024). Table 3 summarizes the effects of ethanol on some neurotrophic factors in different parts of the brain.

Table 3.

Effects of ethanol on some neurotrophic factors in different parts of the brain

Tissue	BDNF	GDNF	CNTF	NGF	NTF-3	NTF-4	Trk-B	MAPK	Ref
Cerebellum	↓						↓		(Climent et al. 2002)
Medial prefrontal cortex	↓								(Haun et al. 2018)
Frontal cortex and Hippocampus	↑								(Kolik et al. 2019)
Primary striatal neurons	↓						↓	↑	(Logrip et al. 2008)
Frontal cortex, Nucleus Accumbens, Amygdala and Hippocampus	↓								(Raivio et al. 2012)
Hippocampus, Septum and Cerebellum	↓			↓	↓				(Heaton et al. 2000)
Cerebral cortex	↓	↓		↓	↓	↓	↓	↓	(Fattori et al. 2008)
Hippocampus				↓					(Walker et al. 1992)
Cortex of neonatal rat pups	↕						↑		(Heaton et al. 2003b)
Cortex	↓	↓/ ↔		↔	↔				(Tsuji et al. 2008)
hippocampus	↓								(MacLennan et al. 1995)
hippocampus	↓	↓		↓					(Song et al. 2015)

BDNF Brain-derived neurotrophic factor, GDNF Glial cell line-derived neurotrophic factor, CNTF Ciliary neurotrophic factor, NGF Nerve growth factor, NTF Neurotrophin, Trk Tropomyosin receptor kinase B, MAPK Mitogen-activated protein kinases. ↓: Decrease; ↑: Increase

Ethanol-Induced Apoptotic Cascades and Their Role in Mood Regulation

Chronic ethanol consumption is associated with a multitude of neuropsychiatric disorders, with depression being a predominant outcome. One critical mechanism through which chronic alcohol use contributes to depression is the induction of apoptosis, a form of programmed cell death that plays a vital role in maintaining cellular homeostasis and regulating neuronal populations. This section delves into the molecular mechanisms by which chronic ethanol consumption leads to increased apoptosis in neuronal cells and how this process contributes to the development of depressive disorders.

Apoptosis is characterized by a series of tightly regulated cellular events that culminate in the death of a cell without eliciting an inflammatory response. This process is essential for normal brain development and function, allowing for the removal of damaged or unnecessary cells. However, in the context of chronic ethanol consumption, the dysregulation of apoptotic pathways can lead to excessive neuronal loss, which is implicated in the pathophysiology of depression (Hicks et al. 2012). Ethanol exposure has been shown to trigger apoptotic signaling cascades in various brain regions, including the hippocampus and prefrontal cortex, which are critical for mood regulation and cognitive function (Radi et al. 2014).

One of the key pathways involved in ethanol-induced apoptosis is the mitochondrial pathway. Ethanol metabolism produces ROS, which can lead to oxidative stress within neuronal cells (Ramachandran et al. 2003). This oxidative stress damages mitochondrial membranes, resulting in the release of pro-apoptotic factors such as cytochrome c into the cytosol. The release of cytochrome c activates, leading to the subsequent activation of downstream effector caspases, primarily caspase-3. Activation of caspase-3 is a hallmark of apoptosis, as it triggers the cleavage of various cellular substrates, ultimately leading to cell death (Han et al. 2005; Young et al. 2005; Zhou et al. 2001). The role of mitochondrial dysfunction in ethanol-induced apoptosis underscores the importance of cellular energy homeostasis in regulating neuronal viability.

Chronic ethanol exposure can activate the extrinsic apoptotic pathway in addition to the mitochondrial pathway. This pathway is initiated by the binding of death ligands, such as TNF-α and Fas ligand (FasL), to their respective receptors, leading to the recruitment of adaptor proteins and the formation of the death-inducing signaling complex (DISC). Ethanol has been shown to increase the expression of TNF-α and FasL in the brain, thereby enhancing the activation of the extrinsic apoptotic pathway. The recruitment of initiator caspases, such as caspase-8, to the DISC, activates downstream effector caspases, including caspase-3, further promoting apoptosis (Svandova et al. 2017). The interplay between the intrinsic and extrinsic apoptotic pathways highlights the multifactorial nature of ethanol-induced cell death in the context of chronic alcohol consumption.

A critical aspect of apoptosis in chronic ethanol consumption is the involvement of neuroinflammation. Chronic alcohol use activates microglia, leading to neuroinflammation characterized by the release of pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6 (Tiwari and Chopra 2012). These cytokines can potentiate apoptotic signaling pathways by activating stress-responsive signaling cascades, including the JNK and p38 MAPK pathways. For instance, TNF-α can activate the JNK pathway, which can lead to the phosphorylation and activation of pro-apoptotic proteins such as c-Jun and Bim. The activation of these proteins promotes apoptosis by facilitating the release of cytochrome c from mitochondria and inhibiting the activity of anti-apoptotic proteins such as Bcl-2 (Aroor and Shukla 2004). The synergistic effects of ethanol-induced neuroinflammation and apoptosis underscore the importance of understanding these interconnected pathways in the context of depression.

The role of endoplasmic reticulum (ER) stress in ethanol-induced apoptosis is another critical area of investigation. Chronic ethanol consumption can lead to the accumulation of misfolded proteins in the ER, resulting in a condition known as ER stress (Chen et al. 2008). This stress triggers the unfolded protein response (UPR), which aims to restore ER homeostasis. However, if the stress is prolonged or severe, the UPR can activate apoptotic pathways. The activation of specific proteins, such as C/EBP homologous protein (CHOP), plays a crucial role in mediating apoptosis during ER stress. CHOP can promote the expression of pro-apoptotic factors while inhibiting anti-apoptotic signals, ultimately tipping the balance in favor of cell death (Yang and Luo 2015). The link between ER stress, ethanol metabolism, and apoptosis highlights the multifaceted nature of the cellular response to chronic alcohol exposure.

Moreover, the impact of chronic ethanol consumption on neurotrophic factors, particularly BDNF, is significant in the context of apoptosis and depression. BDNF is essential for neuronal survival, growth, and plasticity. Chronic alcohol consumption has been shown to decrease BDNF levels, which can exacerbate the vulnerability of neurons to apoptotic signals (Correia and Vale 2024; Hauser et al. 2011). Reduced BDNF levels can impair the signaling pathways that promote cell survival and synaptic plasticity, further contributing to the loss of neuronal integrity (Peregud et al. 2023; Sakai et al. 2005). The interplay between decreased neurotrophic support and increased apoptosis is critical for understanding the development of depressive symptoms in individuals with chronic alcohol use.

The effects of chronic ethanol consumption on the brain’s structural integrity are closely linked to apoptosis. Excessive neuronal loss resulting from increased apoptosis can lead to shrinkage of critical brain regions involved in mood regulation, including the hippocampus and prefrontal cortex. Neuroimaging studies have consistently shown reduced volumes of these regions in individuals with depression, further supporting the notion that apoptotic processes contribute to the observed structural changes associated with mood disorders (Fowler et al. 2014; Hicks et al. 2012). The progressive loss of neurons and synapses can lead to cognitive deficits, emotional dysregulation, and the manifestation of depressive symptoms, reinforcing the connection between chronic ethanol exposure, apoptosis, and depression.

Additionally, the interaction between alcohol consumption and genetic predispositions can influence the susceptibility to ethanol-induced apoptosis and depression. Genetic variations in apoptotic pathways, neurotrophic factor signaling, and inflammatory responses may modulate individual responses to chronic alcohol exposure. For example, polymorphisms in genes encoding for BDNF or components of the apoptotic machinery may predispose certain individuals to heightened neuronal vulnerability and increased risk of developing depression following chronic alcohol use (Peregud et al. 2023). Understanding these genetic factors is essential for identifying individuals at risk and tailoring preventive strategies.

The implications of apoptosis in chronic ethanol consumption extend beyond neuronal loss; they also encompass the broader neurobiological changes associated with mood disorders. The chronic activation of apoptotic pathways can lead to alterations in neurotransmitter systems, including serotonin and dopamine, both of which are critically involved in mood regulation. For instance, apoptosis-induced loss of serotonergic neurons can diminish serotonin availability, contributing to the onset of depressive symptoms. Similarly, the dopaminergic neuronal loss can impair reward processing and motivation, further exacerbating mood dysregulation (Popova et al. 2020). The intricate relationship between apoptosis and neurotransmitter systems highlights the complexity of the neurobiological underpinnings of depression in the context of chronic alcohol use.

Ethanol-Induced Apoptotic Cascades and Their Role in Mood Regulation

Chronic ethanol consumption has profound effects on the central nervous system, leading to significant alterations in neurotransmitter regulation that contribute to the development of depression. Neurotransmitters are essential chemical messengers that facilitate communication between neurons, playing crucial roles in mood regulation, cognition, and various physiological processes. The dysregulation of neurotransmitter systems, particularly serotonin, dopamine, and GABA, is a central mechanism through which chronic alcohol use leads to depressive symptoms. This section will explore the molecular mechanisms involved in neurotransmitter dysregulation due to chronic ethanol consumption and how these alterations contribute to the pathophysiology of depression.

Serotonin is one of the most studied neurotransmitters in the context of depression. It is primarily involved in mood regulation, emotion, and cognition (Arnone et al. 2024). Chronic ethanol exposure has been shown to decrease serotonin levels and disrupt the functionality of serotonergic pathways (Dahchour and Ward 2024). Ethanol primarily affects serotonin regulation by inhibiting tryptophan hydroxylase, the enzyme that controls serotonin synthesis. Ethanol-induced oxidative stress can generate reactive oxygen species (ROS), which may damage tryptophan hydroxylase and impair its function (Qiu et al. 2015). Consequently, this impairment results in decreased serotonin synthesis and reduced availability of this critical neurotransmitter.

Moreover, chronic alcohol consumption can lead to alterations in serotonin receptor expression and function. For example, studies have shown that long-term ethanol exposure can downregulate the expression of serotonin receptors, particularly the 5-HT1A and 5-HT2A receptors (Dahchour and Ward 2024). These receptors are essential for mediating the effects of serotonin in the brain, and their downregulation can diminish the overall serotonergic signaling capacity. Reduced serotonergic signaling is closely associated with the onset of depressive symptoms, as it disrupts mood regulation and emotional processing.

In addition to serotonin, dopamine is another neurotransmitter significantly affected by chronic ethanol consumption. Dopamine is crucial for the brain’s reward circuitry, motivation, and pleasure responses (Wise and Jordan 2021). Chronic alcohol use can lead to dysregulation of dopaminergic pathways, which is often observed in individuals with depression. Ethanol exposure has been shown to reduce dopamine synthesis and release, primarily through its impact on the enzyme tyrosine hydroxylase, the rate-limiting step in dopamine synthesis (Salinas et al. 2021). Ethanol-induced oxidative stress can impair tyrosine hydroxylase activity, reducing dopamine production, similar to serotonin synthesis.

Furthermore, chronic ethanol consumption affects dopamine receptor expression and function. Studies have demonstrated that prolonged alcohol use can lead to a decrease in the density of dopamine D2 receptors, which are critical for the regulation of dopaminergic neurotransmission. The reduction of D2 receptor availability can impair the brain’s reward signaling, leading to anhedonia, a core symptom of depression characterized by a decreased ability to experience pleasure (Salinas et al. 2021). The dysregulation of dopaminergic signaling in the context of chronic alcohol use underscores the importance of dopamine in the development of depression.

The relationship between chronic ethanol consumption and the GABA system is also crucial in understanding neurotransmitter dysregulation and its contribution to depression. GABA is the primary inhibitory neurotransmitter in the brain, playing a critical role in reducing neuronal excitability and maintaining a balanced mood. Chronic alcohol exposure can lead to increased GABAergic activity, particularly in the short term, due to ethanol’s direct action on GABA receptors. However, with prolonged alcohol consumption, the brain adapts to the increased GABA activity, leading to compensatory decreases in GABA receptor density and function (Hansen et al. 2020; Ochoa-de la Paz et al. 2021). This adaptive response can result in GABAergic dysfunction, characterized by reduced inhibitory signaling.

The dysregulation of GABAergic signaling due to chronic ethanol consumption can lead to heightened neuronal excitability and increased vulnerability to stress, both of which are associated with the onset of depression. The balance between excitatory (glutamatergic) and inhibitory (GABAergic) neurotransmission is crucial for maintaining emotional stability, and any disruption in this balance can affect mood regulation. Chronic alcohol use can lead to increased glutamate release and decreased GABA signaling, resulting in a hyperexcitable state that may contribute to anxiety and depressive symptoms (Hansen et al. 2020).

Another important aspect of neurotransmitter regulation in the context of chronic ethanol consumption is the impact on the glutamatergic system. Glutamate is the primary excitatory neurotransmitter in the brain and is involved in various functions, including learning and memory. Chronic alcohol consumption can cause changes in the expression and function of glutamate receptors, particularly the NMDA and AMPA receptors (Wilson and Matschinsky 2020). Ethanol inhibits NMDA receptor activity, which reduces excitatory signaling in the brain. However, prolonged alcohol use can lead to the compensatory upregulation of NMDA receptors, resulting in an excitatory state that may worsen mood dysregulation.

The interplay between the serotonergic, dopaminergic, GABAergic, and glutamatergic systems is complex and highlights the intricate balance required for maintaining emotional stability. Chronic ethanol consumption disrupts this balance, leading to altered neurotransmitter levels and receptor sensitivities that contribute to the development of depressive symptoms. The combined effects of reduced serotonin and dopamine signaling, along with impaired GABAergic and glutamatergic regulation, create a neurobiological environment that predisposes individuals to depression (Wilson and Matschinsky 2020).

Moreover, the dysregulation of neurotransmitter systems in chronic ethanol consumption is often accompanied by changes in neurotrophic factors, which further complicates the relationship between neurotransmission and depression. Neurotrophic factors, such as BDNF are essential for neuronal survival and plasticity. Chronic alcohol exposure has been shown to decrease BDNF levels, which can impair the signaling pathways that regulate neurotransmitter systems. The reduction of BDNF not only affects neuronal health but also further exacerbates the dysregulation of neurotransmitter balance, creating a vicious cycle that can contribute to the onset of depressive symptoms (Peregud et al. 2023).

Stress is another critical factor that interacts with neurotransmitter regulation in the context of chronic ethanol consumption. Chronic stress can lead to alterations in neurotransmitter systems, and individuals who consume alcohol may be more susceptible to stress-induced dysregulation (Dunne and Ivers 2023; Nikbakhtzadeh et al. 2023). The chronic activation of the HPA axis due to stress can lead to elevated levels of cortisol, a hormone known to have neurotoxic effects (Mbiydzenyuy and Qulu 2024). Cortisol can further disrupt neurotransmitter balance by reducing serotonin and dopamine levels while increasing glutamate release (Al-Kufaishi and Al-Musawi 2024). This stress-induced dysregulation compounds the effects of chronic alcohol consumption, leading to an increased risk of developing depression.

The impact of chronic ethanol consumption on neurotransmitter regulation is not only limited to mood disorders but also extends to cognitive function. The alterations in neurotransmitter balance can impair cognitive processes, such as attention, memory, and decision-making, which are often affected in individuals with depression. The interplay between neurotransmitter dysregulation and cognitive deficits highlights the need for a comprehensive understanding of how chronic alcohol exposure affects brain function (Brennan et al. 2020; Garrisson et al. 2021; Mehta et al. 2021; Şahin et al. 2015; Tiwari and Chopra 2012). Table 4 presents a summary of the effects of ethanol on apoptosis in different parts of the brain.

Table 4.

Effects of ethanol on apoptosis in different parts of the brai

Tissue	Bcl2	Bcl-xl	Bax	pAkt	pJNK	Bcl-xs	Caspase 3	TrkB	Ref
Striatum	↓	↓	↑						(Heaton et al. 2003a)
Cerebral cortex	↓						↑	↑	(Climent et al. 2002)
Cortex of neonatal rat pups	↑	↑	↓	↓ then ↑	↑	↓			(Heaton et al. 2003b)
Cortex				↓					(Tsuji et al. 2008)
Forebrain		↓	↑				↑		(Young et al. 2003)
Cerebellum, Brain stem, and Olfactory bulb							↑		(Ieraci and Herrera 2006)
Cerebellum, Brain stem, and Olfactory bulb							↑		(Dikranian et al. 2005)

Bcl-xL B- cell lymphoma extra-large, Bcl-2 B-cell lymphoma-2, Bax bcl-2-like protein 4, Trk Tropomyosin receptor kinase B., ↓: Decrease; ↑: Increase

Disruption in Neurotransmitter Receptor Remodeling

Chronic ethanol consumption leads to significant disruptions in neurotransmitter receptor remodeling, a process that profoundly impacts mood regulation and contributes to the development of depression. Understanding the molecular mechanisms by which ethanol alters neurotransmitter systems is crucial for elucidating how these changes culminate in depressive symptoms. Neurotransmitter receptor remodeling describes the dynamic changes in the expression, sensitivity, and functionality of neurotransmitter receptors in response to various physiological and pathological stimuli, including prolonged exposure to substances such as ethanol.

At the core of the interaction between chronic ethanol consumption and neurotransmitter receptor remodeling is the influence on the GABA system. Ethanol acts as a positive allosteric modulator of GABA-A receptors, thereby enhancing the inhibitory effects of GABA, the primary inhibitory neurotransmitter in the central nervous system. Initially, this modulation promotes feelings of relaxation and euphoria, contributing to the reinforcing properties of alcohol. However, with chronic consumption, the brain adapts to the persistent presence of ethanol by downregulating the expression of GABA-A receptors and altering their subunit composition (Ochoa-de la Paz et al. 2021). This downregulation leads to a decreased inhibitory tone in the brain, resulting in a state of hyperexcitability. The imbalance between excitatory and inhibitory neurotransmission is a critical factor in mood disorders, as it can precipitate anxiety, irritability, and depressive symptoms (Hansen et al. 2020). The compensatory mechanisms that the brain employs in response to chronic ethanol exposure can ultimately exacerbate the underlying neurochemical imbalances, reinforcing the cycle of addiction and mood dysregulation.

The glutamatergic system, which is responsible for excitatory neurotransmission, is also significantly affected by chronic ethanol consumption. Ethanol has been shown to increase the release of glutamate, leading to excitotoxicity, a condition characterized by excessive stimulation of neurons that can cause cellular damage and death. Chronic ethanol exposure remodels glutamate receptor systems, particularly the NMDA receptors critical for synaptic plasticity and cognitive function. Research suggests that NMDA receptor expression may be upregulated as a compensatory response to the excitatory effects of ethanol. However, this alteration can result in harmful consequences, including impairments in synaptic function and increased susceptibility to neuronal injury (Hansen et al. 2020). Changes in the expression and composition of AMPA receptors can disrupt normal excitatory signaling, which is essential for emotional regulation (Gerace et al. 2021). The dysregulation of glutamate signaling in response to chronic ethanol exposure is thus a significant contributor to the emotional and cognitive deficits commonly observed in individuals with AUDs.

Serotonin, a neurotransmitter that plays a crucial role in mood regulation, is significantly impacted by chronic ethanol consumption. Ethanol can alter the expression and function of various serotonin receptors, including the 5-HT2A and 5-HT3 receptors (Dahchour and Ward 2024). Chronic exposure to ethanol can lead to decreased serotonin levels due to its effects on the synthesis and release of serotonin, primarily by influencing tryptophan metabolism. Tryptophan is the precursor for serotonin, and any disruption in its availability can lead to reduced serotonin production (Qiu et al. 2015). This reduction in serotonin signaling is particularly concerning, given the well-established link between serotonin dysregulation and the onset of depressive symptoms. Additionally, the sensitivity of serotonin receptors may also be altered, further complicating the neurochemical landscape in individuals with chronic alcohol consumption. The interplay between serotonin and the HPA axis, which is often dysregulated in individuals with depression, further adds to the complexity of how ethanol-induced changes in serotonin signaling can contribute to mood disorders (Correia and Vale 2024; Dunne and Ivers 2023; Nikbakhtzadeh et al. 2023).

Chronic ethanol consumption also affects norepinephrine and dopamine systems, leading to disruptions in the remodeling of neurotransmitter receptors. Norepinephrine, which regulates arousal and mood, can be influenced by ethanol’s effects on the locus coeruleus, the brain’s primary noradrenergic nucleus. Chronic ethanol exposure can lead to increased norepinephrine release and alterations in adrenergic receptor sensitivity, which may contribute to the anxiety and depressive symptoms often observed in AUDs (Burnham et al. 2021; Fitzgerald 2022). Similarly, the dopaminergic system, which is critical for reward processing and motivation, can be disrupted by chronic alcohol consumption. Ethanol can lead to alterations in dopamine receptor expression and function, impacting the brain’s reward pathways and contributing to the anhedonia and motivational deficits associated with depression (Fitzgerald 2022; Salinas et al. 2021; Wise and Jordan 2021). The maladaptive changes in the dopaminergic system further emphasize the role of neurotransmitter receptor remodeling in the development of mood disorders.

The interplay between oxidative stress and neurotransmitter receptor remodeling adds another layer of complexity to understanding how chronic ethanol consumption leads to depression. Ethanol metabolism generates ROS, which can cause oxidative damage to neuronal cells. Increased oxidative stress can lead to alterations in neurotransmitter receptor expression and function, compounding the effects of ethanol on mood regulation. For instance, oxidative stress has been shown to impair the function of serotonin and dopamine receptors, further exacerbating the dysregulation of neurotransmitter systems (Emerit et al. 2004; Gorlova et al. 2023; Qiu et al. 2015). The chronic state of oxidative stress can create a detrimental environment for neuronal health, impairing the brain’s ability to adapt to stressors and recover from depressive episodes.

Neuroinflammation is another critical factor linked to neurotransmitter receptor remodeling in the context of chronic ethanol consumption. Ethanol can activate glial cells, particularly microglia, which release pro-inflammatory cytokines that can influence neurotransmitter receptor dynamics. The inflammatory environment created by chronic ethanol exposure can lead to changes in the expression and sensitivity of neurotransmitter receptors, further disrupting the delicate balance of excitatory and inhibitory signaling in the brain (Kane and Drew 2021; Kaur et al. 2017; Mikhalitskaya et al. 2024; Richardson et al. 2022; Ruiz et al. 2022; Simkin and Arnold 2020). The interaction of neuroinflammation, oxidative stress, and alterations in neurotransmitter receptors demonstrates the complex nature of ethanol-induced depression, emphasizing the need to understand these connections for effective therapeutic strategies.

The long-term consequences of neurotransmitter receptor remodeling due to chronic ethanol consumption are profound. The persistent alterations in receptor expression and function can lead to neuroadaptation, where the brain becomes increasingly reliant on ethanol to maintain a semblance of homeostasis. This neuroadaptive state can contribute to the development of tolerance, dependence, and withdrawal symptoms, further complicating the recovery process for individuals with AUDs (Bhandari et al. 2024). The chronic dysregulation of neurotransmitter systems can result in a heightened vulnerability to stress and mood disorders, reinforcing the cycle of addiction and depressive symptoms. Table 5 illustrates the changes in neurotransmitters following ethanol administration in animal studies.

Table 5.

This table illustrates the changes in neurotransmitters following ethanol administration in animal studies

Tissue	NE	5-HT	5-HT1A	Dopamine	Dopamine D2/3 autoreceptor sensitivity	Glutamate	NMDA receptors	AMPA receptor	GABA	GABA receptor	Species	Ref
Caudate				↑	↓						Rhesus macaques	(Salinas et al. 2021)
Putamen				↔	↓						Rhesus macaques	(Salinas et al. 2021)
Cortex							↑	↑			Sprague–Dawley	(Chandler et al. 1999)
Hippocampus							↑				Wistar rats	(Staples et al. 2015)
medial prefrontal cortex		↑ / ↓									Wistar–Harlan / Wistar–BgVV	(Langen et al. 2002)
Hippocampus	↔	↑		↔							Wistar rats	(Vasconcelos et al. 2004)
Striatum	↓	↑		↓							Wistar rats	(Vasconcelos et al. 2004)
Striatum				↑							Wistar rats	(Yoshimoto et al. 1992a)
Cerebral Cortex, Cerebellum, Striatum, Limbic Forebrain						↔			↓		Male rats	(Hellevuo and Kiianmaa 1989)
Anterior Raphe Area			↔								Sprague–Dawley rats	(Nevo et al. 1995)
hippocampus			↔								Sprague–Dawley rats	(Nevo et al. 1995)
Striatum			↑								Sprague–Dawley rats	(Nevo et al. 1995)
Dorsal Raphe			↑								Sprague–Dawley rats	(Nevo et al. 1995)
Frontal Cortex			↓								Sprague–Dawley rats	(Nevo et al. 1995)
Dentate gyrus			↓								Sprague–Dawley rats	(Nevo et al. 1995)
Nucleus accumbens				↑							Sprague–Dawley rats	(Imperato and Di Chiara 1986)
Nucleus accumbens				↑							Sprague–Dawley rats	(Rada et al. 2004)
Nucleus accumbens				↑							Sprague–Dawley rats	(Melendez et al. 2003)
Nucleus accumbens				↑							Wistar rats	(Melendez et al. 2003)
Nucleus accumbens				↑							Wistar rats	(Heidbreder and De Witte 1993)
Nucleus accumbens				↑		↓			↓		Male Rat	(Piepponen et al. 2002)
Nucleus accumbens				↑							Wistar rats	(Yoshimoto et al. 1992b)
prefrontal cortex				↑							Sprague–Dawley CD young-adult rats	(Lallai et al. 2016)
prefrontal cortex				↑							Wistar rats	(Ding et al. 2011)
prefrontal cortex				↑							Long–Evans rats	(Schier et al. 2013)

Disruption of Synaptic Remodeling and Plasticity

Chronic ethanol consumption has been shown to have profound effects on synaptic plasticity, which is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. This capacity for change is essential for learning, memory, and overall cognitive function, and it plays a critical role in emotional regulation. Impairments in synaptic plasticity are increasingly recognized as a significant factor contributing to the development of depression, particularly in individuals with a history of alcohol use. In this section, we will explore the molecular mechanisms by which chronic ethanol exposure disrupts synaptic plasticity and how these disruptions can lead to the onset of depressive symptoms.

One of the primary synaptic mechanisms affected by chronic alcohol consumption is LTP, a process that enhances synaptic strength following high-frequency stimulation of a synapse. LTP is crucial for various forms of learning and memory, and its impairment has been linked to several neuropsychiatric disorders, including depression. Ethanol has been shown to disrupt LTP in different brain regions, including the hippocampus, a region critically involved in memory formation and emotional regulation (Yao et al. 2021; Grafe et al. 2021). This disruption occurs through several molecular pathways, including alterations in glutamatergic signaling and changes in receptor expression (Gerace et al. 2021; Nicosia et al. 2024; Yao et al. 2021).

Chronic ethanol exposure can lead to a decrease in the activity of NMDA receptors, which are essential for the induction of LTP. Ethanol inhibits NMDA receptor function, reducing calcium influx into neurons that is necessary for the activation of intracellular signaling pathways involved in LTP (Avchalumov and Mandyam 2020). The reduced calcium signaling impairs the activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a key enzyme involved in the phosphorylation of various substrates that promote synaptic strengthening. Without adequate activation of CaMKII, the processes that lead to LTP are compromised, resulting in diminished synaptic efficacy (Avchalumov and Mandyam 2020). This reduction in LTP is particularly concerning, as it disrupts the neural circuits involved in mood regulation, leading to emotional dysregulation and an increased risk of depression.

In addition to NMDA receptor dysfunction, chronic ethanol consumption also affects the expression and function of AMPA receptors, which mediate fast excitatory neurotransmission in the brain. Ethanol has been shown to decrease AMPA receptor expression and alter their subunit composition, leading to reduced synaptic transmission. This alteration in AMPA receptor dynamics can further contribute to the impairment of LTP, as the activation of AMPA receptors is essential for the stabilization of LTP following its induction (Gerace et al. 2021). The combined effects of NMDA receptor inhibition and AMPA receptor dysregulation create a significant obstacle to the maintenance of synaptic plasticity, which is vital for learning and emotional resilience.

The role of BDNF in synaptic plasticity is also critical in understanding how chronic ethanol consumption impairs these processes. BDNF is a neurotrophic factor that promotes neuronal survival, growth, and synaptic plasticity. Chronic alcohol exposure has been shown to reduce BDNF expression, particularly in the hippocampus and prefrontal cortex, regions heavily implicated in mood regulation and cognitive function. The reduction of BDNF levels negatively impacts the signaling pathways that facilitate LTP, as BDNF is known to enhance NMDA receptor activity and promote the incorporation of AMPA receptors into the postsynaptic membrane (Peregud et al. 2023). Reduced BDNF signaling not only hinders long-term potentiation (LTP) but also encourages long-term depression (LTD), a process linked to the weakening of synaptic connections (Avchalumov and Mandyam 2020). The balance between LTP and LTD is crucial for maintaining synaptic health, and impairments in this balance can lead to cognitive deficits and emotional disturbances.

Moreover, chronic ethanol consumption can induce neuroinflammation, which further exacerbates the impairment of synaptic plasticity. Ethanol activates microglia, the resident immune cells in the brain, leading to chronic neuroinflammation characterized by the release of pro-inflammatory cytokines such as TNF-α and IL-1β. These cytokines can negatively impact synaptic plasticity by inhibiting the signaling pathways involved in LTP. For example, TNF-α has been shown to interfere with the signaling required for NMDA receptor activation and promote the internalization of AMPA receptors, further diminishing synaptic strength (Alvarez Cooper et al. 2020; Montgomery and Bowers 2012). The chronic state of neuroinflammation thus creates an environment that is detrimental to synaptic function and plasticity, contributing to the cognitive and emotional deficits observed in individuals with chronic alcohol use.

The impairment of synaptic plasticity is also closely related to alterations in epigenetic regulation. Chronic ethanol exposure has been shown to induce changes in DNA methylation and histone modifications that affect gene expression related to synaptic plasticity (Legaki et al. 2024). For instance, the promoter region of the BDNF gene can undergo hypermethylation, leading to reduced BDNF expression and subsequent impairment of synaptic plasticity (Motaghinejad et al. 2021; Peregud et al. 2023). Epigenetic modifications can have long-lasting effects on neuronal function, contributing to the persistence of synaptic impairments even after cessation of alcohol consumption. This epigenetic aspect further complicates the pathophysiology of depression in individuals with a history of chronic alcohol use, as the effects of ethanol on synaptic plasticity may endure long after alcohol exposure has ceased.

Additionally, the relationship between chronic alcohol consumption and alterations in the endocannabinoid system is noteworthy in the context of synaptic plasticity. The endocannabinoid system plays a critical role in modulating synaptic transmission and plasticity. Chronic ethanol consumption can disrupt endocannabinoid signaling, which is associated with impairments in both LTP and LTD. The dysregulation of this system can further contribute to the emotional and cognitive deficits observed in depression, as endocannabinoids are implicated in mood regulation and stress response (Kunos 2020). The interplay between ethanol-induced alterations in the endocannabinoid system and synaptic plasticity highlights the complexity of the neurobiological changes associated with chronic alcohol use.

The impact of impaired synaptic plasticity extends beyond the cellular level and manifests in behavioral outcomes. The cognitive deficits resulting from disrupted LTP and LTD can lead to difficulties in learning, memory, and emotional regulation (Baghcheghi et al. 2024). For instance, individuals with depression often experience impaired cognitive flexibility, which is the ability to adapt to changing circumstances and learn from new experiences. This cognitive rigidity results from chronic ethanol exposure’s impact on synaptic plasticity. The inability to form and modify synaptic connections in response to new information can contribute to the persistence of negative thought patterns and emotional dysregulation characteristics of depression (Appelbaum et al. 2023).

Moreover, the effects of impairment in synaptic plasticity are evident in the structural changes observed in the brains of individuals with chronic alcohol use. Neuroimaging studies have shown that chronic alcohol consumption is associated with reductions in hippocampal volume and alterations in the morphology of dendritic spines, which are critical for synaptic connectivity and plasticity (Mira et al. 2020; Amodeo et al. 2021). These structural changes further support the notion that chronic ethanol exposure disrupts the neural circuits involved in mood regulation and cognitive function, ultimately contributing to the onset of depressive symptoms.

Limitations and Future Directions: A Call for Further Research

The exploration of ethanol-induced depression has revealed significant insights into the molecular mechanisms underlying this complex relationship. It’s important to acknowledge that, like any field of scientific inquiry, the current research has inherent limitations. These limitations impact the strength of our understanding and underscore the pressing need for additional research to fill knowledge gaps and improve therapeutic strategies. This section will discuss the limitations present in the existing literature, followed by suggestions for future research directions that could enhance our understanding of the interplay between chronic ethanol consumption and depression.

One of the primary limitations in the field of ethanol-induced depression research is the reliance on animal models to study the effects of alcohol on neurobiology and behavior. While animal models have provided valuable insights into the biological mechanisms of alcohol use and its effects on mood, they often fail to capture the complexity of human behavior and the multifactorial nature of depression. The majority of studies utilize rodents, which exhibit different neurobiological responses to ethanol compared to humans. Moreover, the duration and patterns of ethanol exposure in animal studies may not accurately reflect the chronic alcohol consumption behaviors observed in human populations. This discrepancy can hinder the translation of findings from animal models to clinical settings, underscoring the need for research that incorporates more diverse and representative models, including human studies.

Another significant limitation is the heterogeneity of depression itself. Depression is a multifactorial disorder influenced by numerous biological, psychological, and social factors. Current research often focuses on singular molecular mechanisms, such as neurotransmitter dysregulation or synaptic plasticity impairment, without considering the broader context in which these mechanisms occur. This narrow focus can limit the understanding of how various factors interact to contribute to ethanol-induced depression. Future research should adopt a more integrative approach, examining the interplay between different biological systems, including the neuroendocrine, inflammatory, and genetic factors that may influence individual vulnerability to depression in the context of chronic alcohol use.

Additionally, the variability in individual responses to alcohol presents a challenge in understanding the relationship between ethanol consumption and depression. Genetic predispositions, environmental factors, and personal history all play critical roles in determining how an individual may respond to alcohol and its effects on mood. Current research often overlooks these individual differences, leading to a generalized understanding of ethanol-induced depression that may not apply to all individuals. Future studies should aim to incorporate a more personalized approach, exploring how genetic markers, epigenetic modifications, and environmental factors contribute to the risk of developing depression in alcohol-dependent individuals. This could involve large-scale longitudinal studies that assess genetic and environmental factors in conjunction with alcohol consumption patterns and mood assessments over time.

Furthermore, there is a need for more comprehensive studies on the long-term effects of chronic ethanol consumption on brain structure and function. While existing literature has established a link between alcohol use and alterations in brain morphology, many studies focus primarily on short-term exposure and acute effects. The long-term consequences of chronic alcohol consumption on neural circuits involved in mood regulation and cognitive function remain inadequately explored. Future research should investigate the chronic effects of alcohol on brain structure and connectivity using advanced neuroimaging techniques. This could provide insights into how sustained ethanol exposure alters neuroanatomy and its subsequent impact on mood and behavior, ultimately contributing to a more nuanced understanding of ethanol-induced depression.

Moreover, the role of comorbid conditions in the context of ethanol-induced depression is an area that requires further investigation. Many individuals with AUD also experience other psychiatric conditions, such as anxiety disorders, post-traumatic stress disorder (PTSD), or bipolar disorder. The presence of comorbidities can complicate the clinical picture and may influence the neurobiological mechanisms underlying depression in the context of chronic alcohol use. Future research should explore the interactions between alcohol use and various psychiatric disorders, examining how these comorbidities may amplify or mitigate the effects of alcohol on mood and cognition. Understanding these interactions could inform more effective treatment strategies that address both alcohol dependence and comorbid psychiatric conditions.

In addition to exploring neurobiological mechanisms, future research should also focus on the psychosocial factors that contribute to ethanol-induced depression. Social support, stress, coping mechanisms, and life experiences all play crucial roles in shaping an individual’s response to alcohol and their risk for depression. Investigating how these psychosocial factors interact with biological processes could provide a more comprehensive understanding of the development of depression in the context of chronic ethanol consumption. For instance, studies could assess how social support networks may buffer against the deleterious effects of alcohol on mood or how stress management techniques could mitigate the impact of chronic alcohol use on depressive symptoms.

Furthermore, the impact of gender differences on the relationship between chronic ethanol consumption and depression is an important area for future research. Existing studies have often failed to adequately consider how biological and psychosocial factors may differ between men and women, leading to potential biases in understanding the impact of alcohol on mood disorders. Research has shown that women may be more susceptible to the effects of alcohol on mood and have different patterns of alcohol use compared to men (Li et al. 2020). Future studies should aim to include diverse populations in terms of gender, age, and ethnicity to gain a more comprehensive understanding of how these factors may influence the relationship between alcohol consumption and depression.

Moreover, the exploration of potential therapeutic interventions aimed at mitigating the effects of chronic ethanol consumption on mood is essential. While current treatment options for alcohol use disorder and depression exist, there is a need for more targeted strategies that address the underlying neurobiological mechanisms contributing to ethanol-induced depression. Future research could investigate pharmacological agents that specifically target neurotransmitter systems altered by chronic alcohol use, as well as behavioral interventions that promote neuroplasticity and resilience. For instance, exploring the synergistic effects of combining pharmacotherapy with psychotherapy or mindfulness-based interventions could provide more effective treatment approaches for individuals struggling with both alcohol dependence and depression.

The integration of technology and innovative methodologies in research also presents exciting opportunities for advancing the understanding of ethanol-induced depression. The use of advanced neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), may allow researchers to visualize brain activity and connectivity patterns associated with alcohol consumption and mood disorders. Additionally, the application of machine learning algorithms could aid in identifying biomarkers for vulnerability to depression in individuals with AUDs, facilitating early intervention and personalized treatment strategies.

Finally, there is a critical need for public health research aimed at understanding the broader societal implications of chronic alcohol consumption and its association with depression. Studies should focus on the prevalence of ethanol-induced depression in different populations, the impact of alcohol consumption on mental health at a community level, and the effectiveness of prevention and intervention strategies. By addressing these public health concerns, researchers can contribute to the development of comprehensive policies and programs aimed at reducing the prevalence of alcohol-related mental health issues.

Conclusion

In summary, this narrative review has illuminated the intricate relationship between chronic ethanol consumption and the development of depression, underscoring the multifaceted molecular mechanisms that underlie this complex interplay. By thoroughly examining the evidence, which includes both animal models and clinical studies, we have determined that the effects of prolonged ethanol exposure go beyond just behavioral changes; they significantly impact the biochemical processes in the brain.

The evidence presented highlights how chronic ethanol consumption contributes to the onset of depressive disorders through various pathways, including the escalation of oxidative stress, the promotion of neuroinflammation, and the dysregulation of neurotrophic factors. Additionally, the induction of apoptosis and alterations in neurotransmitter receptor remodeling further complicate the neurobiological underpinnings of ethanol-induced depression. These findings highlight the necessity of a nuanced understanding of these mechanisms and underscore the critical role of neurotransmitter regulation in influencing mood and emotional well-being.

Despite the significant strides made in understanding these processes, several limitations persist in the current body of literature. The need for further research is paramount; specifically, studies that bridge the gap between preclinical findings and human clinical outcomes. Future investigations should aim to elucidate the long-term effects of ethanol on brain function and emotion regulation while exploring potential therapeutic interventions that target these molecular pathways.

Ultimately, advancing our understanding of the molecular mechanisms linking chronic ethanol consumption and depression is essential for developing effective treatment strategies. By fostering a more comprehensive approach that integrates findings from diverse research domains, we can pave the way for innovative therapies that address the dual challenges of alcohol use disorder and depressive disorders, ultimately improving the quality of life for those affected. As we move forward, a collaborative effort across disciplines will be vital in unraveling the complexities of ethanol-induced depression and finding meaningful solutions to this pressing public health concern.

Abbreviations

GABA

Gamma-aminobutyric acid

NMDA

N-methyl-D-aspartate

HPA

Hypothalamic–pituitary–adrenal

AUDs

Alcohol use disorders

MDD

Major depressive disorder

CBT

Cognitive-behavioral therapy

IL-6

Interleukin-6

TNF-α

Tumor necrosis factor-alpha

CRH

Corticotropin-releasing hormone

ACTH

Adrenocorticotropic hormone

MRI

Magnetic resonance imaging

PET

Positron emission tomography

SSRIs

Selective serotonin reuptake inhibitors

ROS

Reactive oxygen species

ADH

Alcohol dehydrogenase

ALDH

Aldehyde dehydrogenase

SOD

Superoxide dismutase

NF-κB

Nuclear factor kappa B

IL-1β

Interleukin-1 beta

LTP

Long-term potentiation

LTD

Long-term depression

NLRP3

NOD-like receptor family pyrin domain containing 3

BBB

Blood–brain barrier

BDNF

Brain-derived neurotrophic factor

JNK

C-Jun N-terminal kinase

P38 MAPK

P38 mitogen-activated protein kinase

MAPK

Mitogen-activated protein kinase

CREB

CAMP response element-binding protein

ERK

Extracellular signal-regulated kinase

NGF

Nerve growth factor

NT-3

Neurotrophin-3

SCFAs

Short-chain fatty acids

FasL

Fas ligand

DISC

Death-inducing signaling complex

ER

Endoplasmic reticulum

UPR

Unfolded protein response

CHOP

C/EBP homologous protein

AMPA

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid

CaMKII

Calcium/calmodulin-dependent protein kinase II

PTSD

Post-traumatic stress disorder

fMRI

Functional magnetic resonance imaging

NE

Norepinephrine

Author Contributions

All authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by [Fateme Razazpour], [Mahdiyeh Hedayati-Moghadam], and [Yousef Baghcheghi]. The first draft of the manuscript was written by [Habibeh Mashayekhi-sardoo], and [Zohreh Hakemi]. The photos were designed by [Habibeh Mashayekhi-sardoo]. [Yousef Baghcheghi] and [Mahdiyeh Hedayati-Moghadam] contributed to the editing and conceptualization. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

None.

Data Availability

No datasets were generated or analysed during the current study.

Declarations

Conflict of interest

The authors reveal no relevant financial or non-financial interests.

Ethical Approval

Not applicable (this paper was provided based on research in global databases).

Consent to Participate

Not applicable (this paper was provided based on research in global databases).

Consent for Publication

Not applicable (this paper was provided based on research in global databases).