GABA as an Overlooked Mediator of Metformin Action: GABA-mediated Neuropsychiatric and Hepatoprotective Activities of Metformin

Abstract

Background

According to treatment guidelines, metformin is the first-line medication for type 2 diabetes. The neuropsychiatric and hepatoprotective activities of metformin, mediated by modulation of gamma-aminobutyric acid (GABA) signalling pathways, have recently attracted the attention of researchers.

Summary

The objective of this narrative review is to compile the most recent data from original research on the GABA-mediated effects of metformin in a range of disorders. Original articles were searched from various electronic databases using relevant search terms. According to animal studies on rodent models of anxiety, metformin increases the intracellular level of AMP-activated protein kinase and GABAergic neurotransmission in the hippocampus and medial prefrontal cortex, resulting in anxiolytic effects. Similarly, metformin increased the levels of GABA and lowered the levels of glutamate in the hippocampus of diabetic epileptic rats. This action was associated with reduced neuroinflammation and apoptosis and improved cognitive function and seizure outcomes. Additionally, an open-label clinical trial with people who have Fragile X syndrome demonstrated that metformin is potentially an effective treatment, and it might improve GABA-mediated inhibition of the cortex as a mechanism. Moreover, beyond the neuropsychiatric action in the central nervous system, metformin reduced ferroptosis in a mouse model of hepatic ischemia/reperfusion injury, with GABA identified as a key microbiota-derived metabolite involved in hepatoprotection.

Key Message

Animal and human studies showed that GABAergic signalling pathways are involved in neuropsychiatric and hepatoprotective activities of metformin. However, the available literature is still fragmented and mainly based on animal studies, suggesting the need for more detailed mechanistic studies and human studies.

Introduction

The traditional use of the medicinal herb Galega officinalis to treat symptoms of diabetes in Europe led to the isolation of the bioactive compound with blood glucose-lowering activity, guanidine. ¹ In the 1920s, guanidine derivatives were synthesised, including metformin.^{2, 3} Metformin was introduced as a diabetes medication in France in 1957 and the UK in 1958.^{1, 4} Although initially overshadowed by more potent but toxic biguanides like phenformin, which were withdrawn in the late 1970s due to lactic acidosis, ² metformin was spared and gradually gained acceptance for its unique benefits, including insulin resistance improvement without weight gain or hypoglycaemia risk. ¹ The UK Prospective Diabetes Study in 1998 demonstrated metformin’s cardiovascular benefits, cementing its role as the preferred first-line treatment for type 2 diabetes.^{5, 6}

Metformin is repurposed and being investigated for multiple conditions other than type 2 diabetes, including cancer prevention and adjunct therapy, polycystic ovary syndrome, weight management, cardiovascular disease and type 1 diabetes adjunct treatment, ageing, cognitive impairment and Alzheimer’s disease, renal disease, infectious diseases like tuberculosis and inflammatory and immune-related diseases including COVID-19.^7–11

Nowadays, metformin is being used as the first-line medication to treat type 2 diabetes due to the availability of sufficient clinical trial evidence on its benefit.^{5, 6, 12} Surprisingly, the mechanisms underlying its therapeutic action are complex and are still not fully understood. Previous literature indicated metformin inhibits hepatic glucose production. However, increasing evidence points towards other sites of action that might also have an important role, including the gastrointestinal tract, the gut microbial communities and the tissue-resident immune cells. ⁹

Pancreatic islet beta-cells produce and release gamma-aminobutyric acid (GABA) with insulin. GABA inhibits glucagon secretion by hyperpolarising alpha-cells via type-A GABA receptors. Studies recently reported that islet β-cells also express type-A GABA receptors, and the activation of these receptors increases insulin release. ¹³ Additionally, GABA, in cooperation with insulin, enhances the proliferation and survival of beta-cells through activation of the PI3-K/Akt pathway. Remarkably, GABA promotes B-cell regeneration and reverses diabetes in a mouse model. ¹⁴ Furthermore, GABA inhibits systemic inflammation and cytokine production, so GABA has a promising therapeutic effect in regulating islet cell function, glucose homeostasis and autoimmunity in the management of DM. ¹⁵

The effect of metformin on GABA signalling in the pancreas and the role of GABA in the antidiabetic activity of metformin is an emerging area of interest, given the potential roles of GABA in glucose metabolism. While direct studies on GABA’s role in the antidiabetic effect of metformin are limited, animal and human studies have indicated that alteration of GABA-signalling is involved in the neuronal as well as hepatoprotective actions of metformin.^16–20 Thus, this narrative review aims to summarise the evidence on the role of GABA in the pharmacology of metformin.

Methods

In general, the purpose of this article was to present a narrative review of the potential role of GABA in the pharmacology of metformin. Evidence from original in vitro, animal and clinical studies was evaluated for bidirectional relationships between GABA signalling and the action of metformin. Given the nature of the subject area, it is clear that this literature search cannot be regarded as a systematic review. Multiple search strategies were employed on electronic databases, including PubMed, Scopus and ScienceDirect, using MeSH terms, keywords and title words during the search. The terms used for these searches were as follows: [Metformin OR Oral antidiabetic OR Oral hypoglycaemic] AND [γ-aminobutyric acid OR GABA]; [Metformin OR Oral antidiabetic OR Oral hypoglycaemic] AND [cognitive function OR seizure OR epilepsy OR anxiety OR fragile X syndrome OR hepatic injury OR liver disease]. Reference lists of identified articles were reviewed. The searches were limited to articles published in English. Articles with key findings of interest were considered, and their conclusions were summarised in a narrative review.

Results

In vivo and in vitro studies have demonstrated that GABA signalling pathways are involved in the therapeutic action of metformin, specifically in anxiety, diabetes/seizure-induced cognitive impairment, Fragile X syndrome and hepatic injury (Table 1).^16–20

Table 1. Gamma-aminobutyric Acid (GABA)-mediated Actions of Metformin.

Disorders	Human, Animal or Cell Culture Studies
	Model	Effects	Potential GABA-mediated Mechanisms
Anxiety	Open field-induced and elevated plus maze-induced anxiety in rats; Primary hippocampal neuron cultures	Metformin induced an anxiolytic effect without tolerance	Metformin activates the AMP-activated protein kinase (AMPK)-forkhead box O3a (FoxO3a)-GABAA receptor-associated protein (GABARAP) signalling pathway in the rat hippocampal CA1 region, which promotes the trafficking of GABAA receptors to the neuronal membrane (Figure 1). 16
	Repeated social defeat induced anxiety followed by elevated plus maze test and light-dark test in mice	Anxiety induces a region-specific reduction in AMPK activity in the medial prefrontal cortex (mPFC). Treatment with metformin restored normal AMPK activity in the mPFC and mitigated social stress-induced anxiety-like behaviours. Furthermore, genetic deletion of AMPK in the mPFC not only instigated anxiety in mice but also nullified the anxiolytic effects of metformin.	Brain slice recordings revealed that GABAergic excitation and the resulting inhibitory inputs to mPFC pyramidal neurons were selectively diminished in stressed mice. Metformin-induced AMPK activation heightened the excitability of GABAergic interneurons and their inhibitory outputs to mPFC pyramidal neurons without directly impacting pyramidal neurons. 17
Hepatic ischemia/reperfusion injury (HIRI)	HIRI in mice	Survival rate of HIRI mice increased, recovery of liver pathological structure, reduction in red blood cell aggregation and consumption of liver glycogen, as well as reduction in serum activity of ALT and AST. Decreased HIRI-induced oxidation in the liver.	Metformin mitigated HIRI-induced ferroptosis by reshaping the gut microbiota, with GABA identified as a key metabolite. 18
Diabetes and status epilepticus-induced cognitive impairment	Morris water maze (MWM) test on experimentally induced diabetic epileptic rats	Metformin ameliorated the deterioration in cognitive function in MWM and reduced seizure score of Pentylenetetrazole-induced status epilepticus in diabetic rats	Treatment with metformin significantly increased the hippocampal GABA by approximately 5-fold and decreased hippocampal glutamate by 76% compared to the untreated diabetic epileptic group. Additionally, metformin reduced the rise in inflammatory cytokines, interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α), as well as apoptotic markers nuclear factor-κB (NF-κB) and caspase-3 metabolite. 19
Fragile X syndrome (FXS)	An open-label trial of metformin in children and adults with FXS	This study has demonstrated the safety of metformin in normoglycemic patients with FXS.	Transcranial magnetic stimulation (TMS) data showed an increase in corticospinal inhibition, a hallmark of FXS pathophysiology, mediated by GABAA and GABAB mechanisms. 20

Discussion

AMPK-FoxO3a-GABARAP-GABA Signalling-mediated Anxiolytic Activity

According to Fan et al.’s study, metformin reduces anxiety in rats. ¹⁶ The researchers showed that metformin effectively decreased anxiety-like behaviours without causing tolerance, which is a common limitation of conventional anxiolytics like benzodiazepines. By activating the AMPK-FoxO3a-GABARAP signalling pathway, which facilitates the trafficking of GABA_A receptors to the neuronal membrane of rat hippocampal cells, metformin mechanistically increased the surface expression of GABA_A receptors and improved inhibitory synaptic transmission (Figure 1).

Figure 1. Metformin Activates the AMP-activated Protein Kinase (AMPK)-Forkhead Box O3a (FoxO3a)-GABA_A Receptor-associated Protein (GABARAP) Signalling Pathway to Promote the Trafficking of GABA_A Receptors to the Neuronal Membrane.

GABA-mediated anxiolytic effect of metformin was also reported by Zhang et al. ¹⁷ This study discovered that anxiety lowers AMPK activity in inhibitory GABAergic neurons of the mPFC, resulting in reduced inhibitory GABAergic input to the medial prefrontal cortex (mPFC) pyramidal neurons of mice. The study showed that metformin specifically activates AMPK in inhibitory GABAergic neurons of the mPFC of mice. Increased level of AMPK leads to increased GABAergic neurotransmission, resulting in notable anxiolytic effects. Moreover, genetic deletion of AMPK in mPFC GABAergic neurons of mice resulted in anxiety-like behaviours and significantly removed the anxiolytic effect of metformin, demonstrating the unique role of AMPK in the GABAergic system in relation to anxiety. According to these results, metformin and other AMPK activators may be effective treatments for anxiety disorders. However, this study did not explain the mechanism by which AMPK increases GABAergic neurotransmission.

Reduced GABAergic neurotransmission is involved in the pathophysiology of psychiatric disorders, such as major depressive disorder, ²¹ bipolar disorder ²² and schizophrenia, ²³ suggesting a promising new therapeutic use of metformin for these psychiatric disorders via regulation of GABAergic signalling in the brain.

GABA-mediated Neuroprotective and Cognitive Benefits of Metformin in Diabetic-epileptic Rats

In diabetic epileptic rats, metformin primarily restores the level of GABA, which confers neuroprotective and cognitive benefits, according to an animal study by Mohamed et al. ¹⁹ According to this study, metformin treatment significantly replenished hippocampal GABA levels that were disrupted by hyperglycaemia and seizures. Together with a decrease in glutamate excitotoxicity, this GABA restoration (restoration of glutamate/GABA balance) helped to enhance spatial memory function and lessen the intensity of seizures. Metformin also reduced the hippocampal IGF-1 levels, resulting in decreased apoptotic markers (NF-κB and caspase-3) and decreased neuroinflammation (IL-1β and TNF-α). However, a study by Chen et al. ²⁴ suggested that metformin enhances excitatory synaptic transmission by increasing glutamatergic rather than GABAergic transmission in hippocampal CA1 pyramidal neurons of healthy mice as a mechanism of neuroprotection. This disagreement may be due to differences in experimental models (diabetic-epileptic rats vs healthy mice) and focus of analysis (neuroinflammatory and neurotransmitter outcomes vs synaptic physiology).

Gamma-aminobutyric Acid-mediated Corticospinal Inhibition in Fragile X Syndrome

Using transcranial magnetic stimulation (TMS), an open-label human study showed that metformin enhanced GABA_A- and GABA_B-mediated cortical inhibition, which is a fundamental deficit in the pathophysiology of FXS. ²⁰

Experimental research in animal models of FXS has shown that metformin is an effective treatment for FXS, as it can improve both cognitive and behavioural phenotypes of FXS in mice and Drosophila models.^25–27 mTORC1 and extracellular signal-regulated kinase (ERK) pathways are hyperactivated in the brains of FXS patients and Fmr1 knockout (KO) mice.^{28, 29} Reduced levels of phosphorylated mitogen-activated protein kinase, ERK and EIF4E, as well as total levels of matrix metalloproteinase 9 (MMP-9) in the prefrontal cortex and hippocampus of mice, suggested that metformin inhibits ERK and mTOR signalling. ²⁵

Using TMS, an open-label human study showed that metformin enhanced GABA_A- and GABA_B-mediated cortical inhibition, which is a fundamental deficit in the pathophysiology of FXS. ²⁰ A study using paired-pulse TMS reported that individuals with FXS, in comparison to healthy controls, displayed reduced GABA_A-mediated inhibition, which is a plausible mechanism underlying cortical hyperexcitability in patients with FXS. ³⁰

Additional mechanistic studies are needed to determine whether metformin’s effects on synaptic signalling (ERK/mTOR) and GABAergic inhibition are mutually reinforcing or context-dependent, especially across species and stages of FXS.

Gut Microbiota-derived Gamma-aminobutyric Acid-mediated Hepatoprotective Activity

By altering the gut microbiota and particularly boosting the number of bacteria that produce GABA, metformin dramatically lowers hepatic ischemia/reperfusion injury (HIRI) in mice, according to a study by Wang et al. ¹⁸ This study demonstrated that metformin-modified faecal microbiota transplantation to treatment-naïve mice has hepatoprotective activity, and gut bacteria eradication with antimicrobials nullifies the hepatoprotective activity of metformin. In line with this study, studies have shown that GABA has hepatoprotective activity in animal models of liver injury.^31–35 Treatment with GABA protected against severe acute liver injury via GABA-mediated STAT3 signalling in mice, ³¹ and decreased TLR4/NF-κB signalling in growing-finishing pigs. ³⁵

An in vitro study in the GLUTag cell line (a model of intestinal L-cells) showed expression of GABA_A, GABA_C and glycine receptors by intestinal L-cells, and the study suggested that GABA and glycine stimulate electrical activity and GLP-1 release from GLUTag cells by ligand-gated ion channel activation. ³⁶ Thus, metformin-induced alteration in gut microbiota-derived GABA may also contribute to its antidiabetic activity, though it requires confirmation with studies. Faecal transplantation from mice treated with metformin to treatment-naive diabetic mice improved glucose tolerance, supporting the concept that the composition of the gut microbiota is relevant to glucose-lowering by metformin. ³⁷ Moreover, another study demonstrated that GABA produced by the gut microbiota, mainly in the colon, alleviates anxiety behaviour via GABA receptors expressed in intestinal epithelial cells without being transferred to the blood. ³⁸

Mechanistic Controversies and Future Perspectives

Studies have shown that metformin has effects on the brain, and its effects on the brain go beyond just controlling blood sugar levels. One key area of focus is on how metformin affects the way brain cells communicate with each other through GABA pathways. Researchers have reported that metformin increases the level of AMPK and GABA signalling in parts of the brain, hippocampus and medial prefrontal cortex.^{16, 17} This action reduces anxiety-like behaviour in a rodent model of anxiety,^{16, 17} and decreases neuroinflammation and improves cognitive problems in diabetic-epileptic rats. ¹⁹ Moreover, the potential effectiveness of metformin in alleviating behavioural and neuropathological deficits associated with neurodegenerative disorders, including Alzheimer’s disease, Parkinson’s disease and Huntington’s disease, is supported by other studies.^39–45 There is also growing evidence that metformin seems to enhance GABA-mediated cortical inhibition in parts of the brain to improve neuropsychiatric symptoms of Fragile X syndrome. ²⁰ However, the current literature on this topic is still fragmented and mainly focused on preclinical evidence. Most studies were conducted using isolated disease models, and it is not well explained how AMPK activation, GABAergic neurotransmission, glutamatergic balance and neuroimmune regulation all tie together yet. Moreover, metformin is actually found to boost excitatory glutamatergic transmission instead of the inhibitory GABAergic signalling in healthy mouse neurons. ²⁴ This shows that there might be context-dependent effects linked to factors like disease state, brain region, dosage or metabolic condition. These inconsistencies indicate the need for more detailed mechanistic studies, not just describing links between metformin and GABA signalling.

The other key area of research is how metformin interacts with gut microbiota and the gut-brain-liver axis. A study on rodents demonstrated that metformin reshapes gut microbiota and enhances microbial GABA production, which protects the liver from ischemia/reperfusion damage by preventing ferroptosis. ¹⁸ Earlier studies support this too, showing that GABA signalling inhibits inflammatory pathways like TLR4/NF-κB while promoting cell-protective STAT3 pathways in liver injury models.^{34, 35}

In addition, microbiota-derived GABA has been implicated in modulation of anxiety-related behaviour through intestinal epithelial signalling pathways without direct blood–brain barrier transport. ³⁸ The link between microbial metabolites and both hepatic and neurobehavioral regulation broadens our understanding of metformin’s pharmacological actions. However, these advances have not eliminated considerable knowledge gaps. Studies in humans assessing microbiota-derived GABA in metformin-treated patients are lacking, causal mechanisms remain poorly defined and the contribution of peripheral versus central GABA signalling is still unclear. In addition, inter-individual variation in gut microbiome composition may have a large impact on therapeutic responses to metformin, ⁴⁶ which may partially explain inconsistent clinical outcomes seen across studies. Therefore, future investigations should combine microbiome analysis, metabolomics, neuroimaging and electrophysiological techniques to determine if GABAergic modulation constitutes a primary therapeutic mechanism of metformin or a secondary consequence of more general metabolic and anti-inflammatory effects.

Conclusion

Through AMPK activation, metformin increases GABA signalling in the prefrontal cortex and hippocampus, which contributes to its anticonvulsant, anxiolytic and cognitive-enhancing properties. It also changes the composition of the gut microbiota, which increases the production of GABA and supports its hepatoprotective effect in HIRI. Together, these results highlight the complex pharmacology of metformin and suggest that GABA modulation is one likely fundamental mechanism underlying its therapeutic adaptability. More mechanistic and clinical studies are needed to assess GABA as a target to extend metformin’s therapeutic application in neurological and hepatic diseases.

Abbreviations

AMPK: AMP-activated protein kinase; EIF4E: Eukaryotic translation initiation factor 4E; FoxO3a: Forkhead box O3a; FXS: Fragile X syndrome; GABA: Gamma-aminobutyric acid; GABARAP: GABA_A receptor-associated protein; HIRI: Hepatic ischemia/reperfusion injury; mPFC: Medial prefrontal cortex; mTOR: Mammalian target of rapamycin; STAT3: Signal transducer and activator of transcription 3; TLK4: Toll-like receptor 4.

Author’s Contribution

This review article was designed and written by YMB. No AI tools were used to generate or analyse scientific content, and the author takes full responsibility for the content of this publication.

Data Availability

Data sharing does not apply to this article as no data sets were generated or analysed during the current study.

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