Potential Differences in Psychedelic Actions Based on Biological Sex

Abstract

The resurgence of interest in psychedelics as treatments for psychiatric disorders necessitates a better understanding of potential sex differences in response to these substances. Sex as a biological variable (SABV) has been historically neglected in medical research, posing limits to our understanding of treatment efficacy. Human studies have provided insights into the efficacy of psychedelics across various diagnoses and aspects of cognition, yet sex-specific effects remain unclear, making it difficult to draw strong conclusions about sex-dependent differences in response to psychedelic treatments. Compounding this further, animal studies used to understand biological mechanisms of psychedelics predominantly use one sex and present mixed neurobiological and behavioral outcomes. Studies that do include both sexes often do not investigate sex differences further, which may hinder the translation of findings to the clinic. In reviewing sex differences in responses to psychedelics, we will highlight the direct interaction between estrogen (the most extensively studied steroid hormone) and the serotonin system (central to the mechanism of action of psychedelics), and the potential that estrogen-serotonin interactions may influence the efficacy of psychedelics in female participants. Estrogen influences serotonin neurotransmission by affecting its synthesis and release, as well as modulating the sensitivity and responsiveness of serotonin receptor subtypes in the brain. This could potentially influence the efficacy of psychedelics in females by modifying their therapeutic efficacy across menstrual cycles and developmental stages. Investigating this interaction in the context of psychedelic research could aid in the advancement of therapeutic outcomes, especially for conditions with sex-specific prevalence.

Traditionally, male participants have been predominantly favored over female participants in studies across species in the name of “simplicity”; that is, the ability to reduce costs and concerns about hormone-related confounding effects (1). Despite the expectation to incorporate sex as a biological variable (SABV) into research practice both in human and animal studies (2), there persists an imbalance between male and female participants in the majority of research studies. Since the implementation of these policies by the National Institutes of Health and the US Congress in 2015 and 2016 aimed at boosting female participation in clinical research, and by December 31, 2019, only 44% of recruited participants were women (3). In neuropsychopharmacology research conducted before 2020, just 20% of studies effectively examined sex differences, and among those, 72% identified statistically significant variations between sexes (4). SABV significantly influences molecular and cellular processes, health conditions, disease outcomes, as well as the efficacy of medications. As an example, the consequences of neglecting SABV in the development of medications for psychiatric conditions, such as selective serotonin reuptake inhibitors (SSRIs), a typical type of serotonergic antidepressant, include increased risk of sexual dysfunction (5, 6), weight gain (7), and increased risk of miscarriage and birth defects (8, 9) in women. Additionally, it is crucial to consider the biological factors influencing mental health conditions to avoid repeating past mistakes of neglecting sex differences in the prevalence of psychiatric conditions that may be treatable by psychedelics.

 

Brief History of Psychedelics

Psychedelics are defined as organic and synthetic substances such as psilocybin and lysergic acid diethylamide (LSD) that exert their effects largely through serotonergic (5-hydroxytryptamine, 5-HT) agonism, which eliminates substances like ketamine and cannabis, which act primarily through N-methyl-d-aspartate (NMDA) and endocannabinoid receptors, respectively (10-12). The discovery of LSD in 1947 by Albert Hofmann coincided almost precisely with the isolation and identification of 5-HT and prompted immediate speculation about the potential for this potent psychoactive compound to interact with 5-HT systems and modulate behavior (13). Consequently, it sparked curiosity and interest among scientists and psychiatrists about the potential utility of psychedelics as “new tools for shortening psychotherapy” (14). This occurred at a period when psychiatry lacked effective medical treatments (15) and during a time when potential sex differences in both the presentation of psychiatric symptoms and the efficacy of novel treatments were often overlooked (16-19). However, the importance of understanding SABV in response to psychedelics as potential therapeutic treatments is brought into sharper focus by the recent resurgence of psychedelic research, in light of what is now known about symptom variability between sexes and differences in medication efficacy.

Sex Differences in Psychiatric Conditions

Steroid hormones emerge as the foremost consideration when contemplating sex differences among humans and rodents. The natural fluctuations of steroid hormones during different reproductive stages in a woman's life contribute to a heightened susceptibility to mood disorders like premenstrual dysphoric disorder, postpartum depression, and perimenopausal depression, all of which are driven, in part, by disruptions to 5-HT signaling (20-22). Furthermore, fluctuating levels of endogenous estrogen and progesterone influence specific serotonin receptor (5-HTR) subtype activity and/or expression, which regulate the mechanism of action of psychedelics and are commonly impaired in various psychiatric disorders (23-25). Among the psychiatric conditions currently being studied in psychedelic research, the heightened prevalence of anxiety and depression in women is linked to the fluctuation of hormones such as estrogen, progesterone, and oxytocin (26-28). Similarly, eating disorder (ED) symptoms are particularly prevalent among female adolescents and young women, posing a considerable risk for a severe and enduring prognosis, with the global health burden being skewed toward females (29). Sex hormones as well as sex differences in brain regions associated with aspects of learning and cognition may contribute to the development and persistence of EDs among females (30). Moreover, the historical disparity between men and women in substance use disorders (SUDs) is decreasing. For example, more women than ever before are experiencing dependence on alcohol (31, 32). In addition, women might exhibit a higher vulnerability to experiencing cravings and relapses (33, 34) as well as more severe adverse medical, psychiatric, and functional consequences associated with SUDs (31).

All these factors underscore a need to thoroughly examine sex disparities within mental health conditions across molecular, neural circuit, and system levels. Additionally, they emphasize the importance of using males and females in mechanistic studies in animal models to effectively translate new fundamental knowledge to inform the application of psychedelic treatments in the clinic. Sex differences are well known to exist in animal models used to understand the biological drivers of the aforementioned psychiatric disorders, yet there is a notable scarcity of studies using male and female animals to assess differential responses to psychiatric drugs between the sexes (35). While animal models often effectively mimic drug responses in both sexes, there is a lack of consistency in the parameters for testing between male and female rodent studies (35) and the translational relevance of common simple behavioral paradigms is sometimes questionable.

Sex-Dependent Variations in Drug Absorption and Efficacy

Factors such as body weight, plasma volume, plasma protein levels, and drug transporter function likely influence the pharmacokinetic variability (bioavailability, distribution, metabolism, and elimination) seen between men and women (36). Women experience slower drug absorption, metabolism, and excretion, resulting in elevated plasma levels that can increase the risk of unwanted side effects (37). Body composition and hormonal transitions also affect receptor binding and sensitivity, as well as the drug's receptor binding profile (38-41). Female sex hormones, particularly estrogen, play a significant role in the sex-specific variations observed in the pharmacodynamics of drugs (36), especially those targeting the central nervous system, which are relevant for many psychiatric conditions and particularly psychedelic therapies. Furthermore, the metabolism of drugs by the liver is influenced by factors such as cardiac output and liver blood flow, both of which are typically lower in women compared to men (42). The expression and regulation of enzymes and transporters required for metabolism of drugs (42-44) and renal elimination of drugs are slower in women compared to men (44). This includes frequently prescribed medications like gabapentin, pregabalin, aminoglycosides, cephalosporins, fluoroquinolones, and vancomycin (45, 46). It is therefore not surprising that sex differences are also evident in medications for psychiatric disorders like SSRIs, with females exhibiting a more favorable response compared to males (47), although this is more often associated with dose-related adverse drug reactions (48). Similarly, antipsychotic medications lead to more significant improvements but more pronounced adverse drug reactions in women due to their higher dopamine receptor engagement compared to men at equivalent serum levels (36). Estrogen enhances dopamine sensitivity and therefore questions the adoption of treatment guidelines based on studies in men for female patients. Indeed, this might explain the adverse effects of antipsychotic medication in women during times of elevated sex hormone levels, such as during ovulation and gestation (36, 37). Estrogen also affects both the production of 5-HT and transporter binding (49), which may directly affect the efficacy of SSRIs across cycle stage (50, 51). This effect is established as older women respond poorly to SSRIs, but hormone replacement therapy can eliminate this poor response (52).

In the rapidly evolving field of psychedelics as treatments for mental health conditions, it is crucial to avoid repeating past mistakes and to ensure a thorough consideration of sex differences in attempts to understand their therapeutic mechanisms. Therefore, there is a need to understand the behavioral, neurobiological, and endocrine effects of different clinically relevant psychedelic compounds to improve treatment efficacy. This is especially pertinent considering the known actions of psychedelics on the 5-HT system and the interactions between 5-HT and estrogen signaling (53). This review will concentrate on studies carried out during the modern resurgence of psychedelic research in the early 21st century. Our aim is to emphasize the scarcity of studies explicitly comparing effectiveness between males and females, with the view to advocate for the inclusion of both sexes in future research endeavors.

Known Actions of Psychedelics in the Human Brain Largely Derive From Studies in Men

A reliable evaluation of sex differences in response to psychedelics in human brain imaging studies is challenging, considering that many psychedelic studies exclusively enroll male participants (54), or sex ratios are not disclosed (55-57). Of those including women, they comprise an average of only 33% of experimental cohorts (58-66). Regardless, psilocybin shows promise in terms of safety and efficacy in patients facing end-of-life anxiety as well as those suffering from treatment-resistant depression, with notable reductions in depressive mood and anxiety observed both in the short- and long-term (67-71). In a recent study involving individuals with anorexia nervosa (AN), psilocybin was found to be safe and well-tolerated in females with a majority of participants reporting the treatment as deeply meaningful, resulting in positive life changes (72). However, the effect on ED symptoms varied significantly, with some individuals showing substantial improvement after a single dose, whereas others did not (72). In healthy individuals, psychedelics modulate neuronal connectivity in the amygdala, potentially shifting toward positive emotional states by reducing the influence of cognition over emotion processing (59, 60, 62, 63, 66), but whether there are nuanced differences in the effects of psychedelics on brain function and/or behavior in males and females is still largely unexplored. Although no differences in the therapeutic efficacy of psychedelics have been described between sexes (Table 1), SABV did not predict acute responses to psilocybin in an early meta-analysis in healthy volunteers (73), which perhaps gave subsequent clinical studies latitude to overlook exploring potential differences based on biological sex. In addition, no differences have been observed in the pharmacokinetic parameters of orally ingested LSD (74), subjective effects of LSD (75), or LSD-induced increases in plasma glucocorticoids between healthy men and women (76). However, these findings in healthy humans were observed in small samples (n = 8/sex) and do not negate the possibility that physiological and psychological sex differences may occur in response to psychedelics in patient populations.

Table 1.

Registered psychedelic clinical trials in different mental disorders

Diagnosis	Treatment	Location, if available	No. of studies registered	Sex	Exclusion criteria
Depression, MDD, TRD	Psilocybin	US, Germany, Canada, UK, Sweden, New Zealand, France, Switzerland, Czechia	52	Mixed	Pregnant, trying to get pregnant, nursing, or unwilling to use effective contraception methods (eg, hormonal or barrier methods, or abstinence) during entire duration of study. Sexually active women and men capable of bearing children who do not consent to use an approved contraceptive method for duration of their involvement in study. Family history of psychotic disorders. Prior or current use of various medications, including those used to treat condition of interest (eg, SSRIs).
	LSD	Switzerland	2
Anxiety	Psilocybin	US, Canada, Jamaica	10
	LSD	Switzerland	3
OCD	Psilocybin	US, UK	6
SUD	Psilocybin	US, Denmark, Canada, Switzerland, France	22
	LSD	Switzerland	1
AN	Psilocybin	US, UK	4	Mixed, females only

Information obtained from ClinicalTrials.gov.

Abbreviations: AN, anorexia nervosa; LSD, lysergic acid diethylamide; MDD, major depressive disorder; OCD, obsessive-compulsive disorder; SSRI, selective serotonin reuptake inhibitor; SUD, substance use disorder; TRD, treatment-resistant depression; UK, United Kingdom; US, United States.

The most commonly reported neurobiological changes following psychedelic administration involve alterations in global connectivity, whether during rest (resting-state connectivity) or task-based imaging, whereby blood flow is increased (77), network-level activity is altered (55, 56, 61, 77), and the net outcome is cortical “desynchronization” (less synchronous neuronal firing) (54). These changes often precipitate the “psychedelic state,” characterized by vivid and lifelike alterations in perception (58), as well as phenomena including “ego-dissolution,” “altered consciousness,” and “altered perception of meaning” (57, 61), which relate to changes in the way one perceives the self and other. These changes are particularly pronounced in brain regions with rich 5-HT2AR subtype expression, whose binding is responsible for the acute “subjective” (hallucinogenic) effects of these compounds (54, 55, 57, 64, 78) and emotional processing (62), along with improvements in neural flexibility (79) and reward processing (80) that may contribute to the robust antidepressant effects of psychedelics (79, 81). On the contrary, one study demonstrated that LSD notably hindered executive functions such as cognitive flexibility and working memory, yet it did not affect the propensity for risky decision-making (82). These contradicting outcomes may stem from the utilization of diverse tasks across these studies, alongside potential disparities in the mechanisms of action among psychedelic substances and doses employed. Nonetheless, clearly there are profound effects of psychedelics on brain function; however, whether these effects differ between men and women is at this stage difficult to determine because of issues related to sample size, exclusion, and inclusion criteria and expectancy biases. Moreover, while psychedelics have demonstrated effectiveness across various diagnoses, it is questionable to consider them as a “universal treatment” for all of these conditions. It is more plausible to propose that the effects of psychedelics are connected to underlying cognitive, behavioral, and neurochemical features that are shared by these conditions. This underscores the importance of studies conducted in animal models, as they enable precise and controlled manipulation of the proposed mechanisms of psychedelics, to understand what is required for therapeutic efficacy across disorder categories.

Utility of Animal Models for Understanding Sex Differences in Response to Psychedelics

Animal models are important for elucidating general biological mechanisms related to psychedelic effects and have already effectively highlighted variations in response patterns between males and females (83, 84). Nonetheless, the majority of recent and influential studies investigating psychedelic mechanisms using animal models have exclusively used male subjects.

Effects of Psychedelics Shown in Males or Female Animals Only

In adult male mice, serotonergic psychedelics such as psilocybin and LSD are shown to improve social reward learning (85) and enhance social behavior, although it is unclear whether effects require one or many doses (86), and whether the neurobiological mechanisms supporting these social changes are related to neural circuit or extracellular membrane function, as both have been described (85, 86). Moreover, psilocybin promotes the rapid extinction of the conditioned fear response in male mice (87, 88) and may be important for posttraumatic stress disorder, at least in males. This was associated with brain-derived neurotrophic factor (BDNF) and mechanistic target of rapamycin (mTOR)-induced hippocampal neuroplasticity. The synthetic psychedelic, 2,5-dimethoxy-4-iodoamphetamine (DOI), reduced inflammation, depression, alcohol use, and contextual fear extinction in male mice (89-91).

The acute subjective effects of psychedelics are linked to the activation of the 5-HT2A receptor subtype (5-HT2AR) (92), thus making these receptors the focal point of much psychedelic research to date. However, our understanding of how psychedelics regulate 5-HT2AR is incomplete, given that these effects have mainly been demonstrated in males. For example, LSD binding to the 5-HT2AR disrupts prepulse inhibition (an index of sensorimotor gating) in male rats and affects prosocial behavior in male mice (86, 93). Psilocin (an active metabolite of psilocybin), on the other hand, produces anxiolytic (90, 94), antidepressive (95), and anticompulsive (96, 97) effects in male mice independent of actions at the 5-HT2AR. An extensive investigation into the effect of psilocybin on the functional connectivity (FC) of male mice throughout the brain has revealed increased FC within 5-HT–associated brain regions, including the striatal, thalamic, and midbrain areas, as well as cortical regions associated with the default mode network. Conversely, FC between DA-related regions and striatal resting-state networks was decreased (98), suggesting widespread neuronal actions of these compounds that do not exclusively involve serotonergic signaling in male mice.

The mechanistic insights gained from animal studies in males is unlikely to be applicable to females, given the large number studies highlighting the mechanisms contributing to sex differences (90, 94, 96, 97). However, a few studies have examined the effects of psychedelics only in female animals by design. This may be because of the smaller size of female rats to fit within a magnetic resonance imaging scanning tube (99) or the reliable development of the phenotype of interest only occurring in adolescent females (100). What these studies demonstrate is that, by and large, mechanisms of psychedelics are similar to those described in males, in which the psilocybin-induced alleviation of compulsive behavior in female mice (101) and cognitive inflexibility in female rats (100) is observed to be independent of actions at the 5-HT2AR. Interestingly, psilocybin affected cognitive flexibility in male and female rats in set-shifting and reversal learning tasks, respectively, which are known to engage different neural circuitry (102). Moreover, females were more sensitive to the anticompulsive effects of psilocybin, suggesting potentially higher efficacy in females for alleviation of compulsive-like behavior. Psilocybin also demonstrated a targeted decrease in network-based hypoconnectivity linked to alcohol use disorder (AUD) in female Wistar rats, primarily driven by medial prefrontal regions (99). The increase in FC between serotonergic core regions and cortical areas observed in female rats in this study was similar to that observed in male mice (98) although with greater specificity in the brain structures involved, possibly due to differences in brain sizes and anesthetic regimens (98, 99). A more robust understanding of the neurobiological and behavioral changes elicited by psychedelics across age, sex, and compound type is needed to keep up with the rapid pace of their clinical application.

Evidence for Sex Differences in Response to Psychedelics in Animal Models

While the aforementioned studies used a single sex for various (mostly unknown) reasons, some key animal studies have used both sexes to highlight aspects of brain function and behavior that are particularly sensitive to psychedelics in males or females. For example, the head-twitch response (HTR) in mice or “wet dog shake” behavior in rats is an acute behavioural readout of psychedelics, which has mixed reports regarding sex differences potentially related to species or strain differences, or the types of psychedelic compounds tested. Older studies in C57BL/6J mice using psilocin and 5-MeO-DMT (the psychedelic compound secreted by the Bufo alvarius toad) showed dose-dependent increases in HTR without differences between sexes (84). The synthetic psychedelic DOI increased prepulse inhibition in male 129S6/SvEv mice but not in females (83), and it was subsequently demonstrated that female C57BL/6J mice exhibit more HTR behaviors compared to males (103), an effect that was not observed in 129S6/SvEv mice (15, 83). Although DOI accumulation in the frontal cortex was similar between the sexes, pharmacokinetic variances were evident, with lower DOI concentrations detected in females. Notably, spontaneous HTR frequencies showed no sex-based differences, indicating that the effect of sex on DOI-induced HTR is contingent on strain-specific factors (9). Psilocin also had no sex-specific effect on wet dog shake behavior in Wistar rats; however, psilocin-induced changes in locomotor activity and disruption in sensorimotor gating were more pronounced in males compared to females, particularly during the proestrus and estrus stages of the reproductive cycle (characterized by high levels of estrogen and progesterone) (104). Another study reported reduced sensitivity to the hypolocomotor effects of LSD in female rats compared to males (105) that was accompanied by cycle-stage dependent disruptions in sensorimotor gating. Collectively, these studies demonstrate the potential for sex to influence the effects of psychedelic treatment; however, the inconsistent results highlight the need for further research.

With respect to the animal studies aimed at understanding the therapeutic mechanisms of psychedelics, impaired cognitive flexibility is observed in multiple psychiatric conditions in which psychedelics are being trialed clinically, including anxiety, AUD, AN, and obsessive-compulsive disorder (OCD). The acute effects of psilocybin significantly enhanced cognitive flexibility both in male and female rats, as observed in a task involving switching between previously learned strategies in response to unexpected changes in the environment (106). A similar outcome was observed in female rats post acutely (100), despite differences in tasks, durations, and accuracy criteria employed in these studies. Conversely, DOI impaired cognitive flexibility, indicating that this particular effect of psilocybin does not extend to all other serotonergic psychedelics (106).There is also evidence of sex differences in response profiles more directly relevant to AUD, whereby psilocybin decreases alcohol preference in male but not female C57BL/6J mice (107). In contrast, neither psilocybin nor LSD had long-lasting effects on relapse-like drinking after alcohol deprivation in either male or female rats (108). A rapid-acting and shorter-lasting derivative of psilocybin, 4-OH-DiPT, reduced fear responses during extinction training in mice with greater efficacy in females than males (109), which was related to a novel mechanism of increasing inhibitory input to the basolateral amygdala. Enhanced fear extinction was seen after chronic intermittent administration of low (“micro”) doses of the psychedelic dimethyltryptamine (DMT) equally in male and female rats, although weight gain was evident only in males (110). Variability in the efficacy of psilocybin and DOI in pain reduction has also been demonstrated in mouse models of chronic pain, with a dose-dependent reduction in pain, lasting up to 14 days after psilocybin and up to 6 hours after DOI, with similar response profiles in both sexes (111).

At the cellular level, a single dose of psilocybin induced rapid and enduring increases in cortical dendritic spine density with one study showing a more pronounced effect in female mice (112), but no sex differences found in a subsequent study (113). Moreover, psilocin triggers sex-specific changes in the activity and responsiveness of the central nucleus of the amygdala (CeA) of Sprague-Dawley rats (114). Since psychiatric disorders affect CeA function (115-118), a decrease in amygdala reactivity or connectivity elicited by psychedelics is associated with favorable therapeutic outcomes in humans (55, 60, 63). In female rats, psilocin administration increases the expression of the immediate early gene, cFos, in the capsular division of the CeA, while males experience long-term reductions in CeA activity lasting up to 28 days after drug administration. These findings suggest that psilocin affects CeA activity differently in males and females, potentially due to variations in plasticity-induced alterations in upstream neural circuitry (114). However, a recent systematic brain mapping study used cFos to identify regionally specific activation of the brain after psilocybin treatment in mice and did not detect any sex-dependent differences, although the sample sizes used were small (n = 4) (119). What is clear from these studies is that there are observable differences between males and females in response to psychedelics that exist at the level of behavior, brain function, and neuronal morphology in animal models (Fig. 1).

Figure 1.

Changes observed in preclinical studies using psychedelics based on biological sex. A summary of the behavioral and neurobiological changes observed after psychedelics in rodent studies using either a single sex or both. These studies have used different compounds, doses, strains, and time courses; however, they provide an initial basis from which to explore underlying mechanisms that may be specific for males or females. Upward arrows indicate psychedelics-induced improvements, while downward arrows indicate an effect in the opposite direction. Larger sizes of sex symbols within the Venn diagram overlap represent the sex in which the effect is most pronounced.

However, it is essential to recognize the sex and strain differences in behavior, brain function, and neuronal morphology in rodents (120-123) and that the sex differences in rodent models of psychiatric disorders being tested for psychedelic treatments are often overlooked or deemed irrelevant to the study of disease neurobiology and treatment responses (35). Additionally, conflicting and sometimes opposing results in the literature highlight one of the barriers to understanding the causes of sex differences: the lack of standardization across studies. This is challenging as research investigating diverse psychedelic compounds with variable receptor binding and molecular signaling properties is often grouped together under the banner of “psychedelic compounds.” Even studies investigating effects of the same compound use different dosing regimens, in animals of different ages, species, and strains. While we appreciate there are multiple challenges to developing a “standard operating procedure” for investigating the mechanisms of psychedelics, one step toward a more translatable body of preclinical work is to incorporate male and female animals in all studies. Despite the knowledge gained from human and rodent studies, we are still far from fully understanding the mechanisms of psychedelics. Variations in the design, conduct, and analysis of experiments, along with the failure to report menstrual or estrous cycles in human and rodent subjects, respectively, undermine the application of SABV in psychedelic research, even when sex differences are reported.

Potential Mechanisms Driving Different Response Profiles to Psychedelics Based on Biological Sex

The interaction between sex hormones (estrogen, progesterone, testosterone) and the 5-HT system could provide a foundational basis to understand how males and females might respond differently to psychedelic treatments. One suggestion is a difference in the distribution or abundance of this 5-HT2AR subtype; however, there are no sex differences in the acute perceptual responses, which depend on the 5-HT2AR, in healthy humans (124), suggesting this is unlikely. However, whether this is true for psychiatric disorders, in which psychedelics are being trialed as treatment options, requires further research. There may be other interactions between psychedelics and sex hormones, acting via the 5-HT system, that need to be elucidated to inform their clinical application. Levels of sex hormones including estrogen and progesterone fluctuate cyclically in humans (∼28 days) (125) and rodents alike, with cycle duration varying between 2 and 8 days in mice (126) and 4 or sometimes 5 days in rats (51). 5-HT and estrogen have a particularly potent interaction, as estrogen can readily cross the blood-brain barrier and affect neuroplasticity and cell excitability, especially during stages of the cycle in which estrogen is high (127). The primary form of estrogen during reproductive years, 17-β estradiol (E2), increases 5-HT levels both by enhancing tryptophan hydroxylase (TPH) production, which is essential for 5-HT synthesis, and by inhibiting transcription of the serotonin reuptake transporter (SERT) gene, thus extending 5-HT availability in synaptic and interstitial spaces to enhance its effects (128, 129). Along with roles of estrogen receptors in modulating the breakdown of 5-HT via the suppression of monoamine oxidase A (MAO) (119), and links between SERT efficacy and changes in binding potential for specific 5-HT receptors in the brain (128, 129), E2 plausibly modulates the efficacy of psychedelics by altering 5-HTR activity (Fig. 2).

Figure 2.

The association between serotonin, estrogen, and psychedelic signaling. The relative peak of estrogen across the estrous cycle in A, rodents, aligns with the highest gene expression of Htr2a (the gene encoding the 5-HT2A receptor) and the lowest gene expression of Htr1a (the gene encoding the 5-HT1A receptor). At the synapse B there are multiple sites of interaction between 5-HT and either estrogen or psilocin that may be responsible for altering the effects of psychedelics based on estrogen availability. Estrogen can bind to either membrane-bound (mER) receptors or permeate the cell membrane to bind to nuclear receptors (ERα/β), to exert its influence on 5-HT transmission. I, Estrogen increases 5-HT levels by enhancing tryptophan hydroxylase (TPH) production, the rate-limiting enzyme that synthesizes 5-HT. II, Inhibition of the transcription of the serotonin reuptake transporter (SERT) gene by estrogen further extends 5-HT availability in the synaptic cleft and interstitial spaces. III, Downstream of this, estrogen also has an inverse effect on levels of the monoamine oxidase A (MAO-A) enzyme targeting the breakdown of 5-HT into its inactive metabolite, 5-HIAA. IV, In combination, this results in alterations in intracellular levels of 5-HT and its transmission across the synaptic cleft. V, Estrogen could also enhance the activity of 5-HT neurons by inhibiting 5-HT1AR signaling, achieved through increased uncoupling of G proteins to 5HT1ARs via activation of the mERs. VI, Estrogen, acting through nuclear receptors (ERα/β) that directly bind to promoter regions at estrogen response element (ERE) sites, enhance 5-HT2A gene transcription and functionally increase neural sensitivity to presynaptic 5-HT. VII, Considering all of these factors and within the framework of psychedelics' mechanisms of action, estrogen plausibly modulates the efficacy of psychedelics by upregulating the expression of 5-HTRs, which in turn increase the plasticity-related gene expression. Figure created with Biorender.com.

5-Hydroxytryptamine, Estrogen, and Psychiatric Disorders

Estrogen regulates the function of multiple brain regions responsible for mood, behavior, and cognition, as well as influencing different neurotransmitters such as 5-HT, DA, glutamate, and γ-aminobutyric acid (GABA) (20, 130-132). Receptors both for estrogen and 5-HT are coexpressed in numerous cell types and across various tissues, suggesting a reciprocal interaction between 5-HT activity and estrogen (53). Moreover, estrogen influences 5-HT receptor activation, as well as synthesis and degradation, all of which serve to regulate neural firing (49). Estrogen primarily acts within the brain via estrogen receptors (ER), which are categorized into 2 identified subtypes: ERα and ERβ (133-135). These receptors can function as intracellular transcription factors by binding to the estrogen response element to initiate transcription, or they can act as membrane-bound receptors (136). ERα is mainly present in specialized regions for reproductive functions both in the body and brain, while ERβ is distributed across various brain regions, particularly the hippocampus and amygdala, which are crucial for learning, memory, and emotional regulation (136-138). ERβ knockout mice show reduced brain 5-HT concentrations, suggesting an important role in the 5-HT system (136, 138). Moreover, E2-induced ERα activity increases 5HT1AR levels, while activation of ERβ upregulates the 5HT2AR (139). ERβ mediates various effects of E2 on the 5-HT system in the brain (140), such as suppressing 5-HTR1AR and elevating 5-HTR2AR gene transcription, which could alter the efficacy of psychedelics (141) in females compared to males, or at different stages of the reproductive cycle. This could also be the case in rodents, as peak estrogen levels during proestrus and estrus align with elevated baseline 5-HT levels (142) (see Fig. 2). This coincident 5-HT1A suppression and 5-HT2A activation is especially intriguing when considering the transient changes in the transcription of these receptors is observed in the cortex of female rats following psilocybin administration (100).

Estrogen administration improves mood and depressive symptoms, particularly in people with low or fluctuating estrogen levels (21), an effect potentially modulated by the 5-HT system through both membrane ERs and nuclear ERs (21, 143, 144). This is further reinforced by the greater predisposition to various psychiatric disorders in people with various ER gene variants (143). The absence of ERβ, rather than ERα, alters BDNF-5-HT2A signaling and synaptic plasticity, thereby increasing the susceptibility to a depressive state (145). This suggests antidepressant and anxiolytic effects of ERβ could be mediated through the regulation of TPH expression and 5-HT production (141, 144, 146). In contrast, ERα tends to exhibit anxiogenic-like properties, although conflicting findings exist regarding its effects (144, 146). In the context of ED, mounting evidence suggests that estrogen protects against the genetic and phenotypic susceptibility to disordered eating in girls and women (147). Estrogen also alleviates obsessive-compulsive symptoms by enhancing 5-HT signaling, an important finding considering altered 5-HT2AR and polymorphisms in SERT are linked to OCD (148, 149). An intriguing explanation for the increased prevalence of AN and other EDs in females involves an abnormal response to estrogen caused by irregularities both in membrane- and nuclear-bound ERs, with genetic variations in genes encoding ERα and ERβ significantly associated with the disorders (150). The actions of estrogen at 5-HTRs and impaired receptor function or SERT expression in individuals with AN or EDs, respectively (128), highlight the clear involvement both of estrogen and the 5-HT system in these individuals. Despite evidence pointing to estrogen as a potential factor in sex differences observed in various psychiatric conditions and the efficacy of psychedelics, with the 5-HT system being the primary mediator, no study to date has addressed the interactions between the estrogen and 5-HT systems in the context of psychedelic research.

Conclusion

Addressing the historical and ongoing sex disparities in research, particularly within the realm of psychedelic studies, is imperative for advancing therapeutic outcomes. From the studies reviewed here, it is evident that psychedelic research is unfortunately following old conventions of neglecting a sex-oriented approach for treating psychiatric conditions. Consequently, we are still far from comprehensively understanding the sex differences observed both in human and rodent studies. So far, clinical psychedelic research shows a recruiting imbalance in male and female participants and there is a clear lack of standardization in understanding the mechanisms of action, which is also seen in animal studies. While the resurgence of psychedelic research promises new therapeutic avenues, especially for conditions with sex-specific prevalence, incorporating SABV into research design is crucial. Likewise, the role of estrogen in observed sex differences associated with psychedelics remains neglected. Given the interactions between 5-HT and estrogen, the importance of sex hormones at various ages and ovulatory status cannot be overlooked in developing effective therapeutic approaches for women, particularly true for psychedelics that have known 5-HT based mechanisms. Estrogen, in particular, may be the most pertinent factor contributing to the sex differences observed (albeit yet to be fully ascertained) in the effects of psychedelics.

Incorporating SABV into research methodologies poses difficulties and hurdles. Traditional scientific practices typically overlook SABV, necessitating an adjustment period for widespread adoption of these new approaches. Addressing ethical concerns, managing increased complexity in study designs, higher expenses, and the requirement for larger sample sizes are all challenges that must be tackled to advance personalized and sex-specific therapeutic strategies in psychedelic research. Therefore, an essential first step is conducting inclusive investigations involving female participants across different menstrual cycle stages and age ranges to comprehensively understand the role of estrogen in the efficacy of these substances. By including men and women in clinical and preclinical studies with experimental designs that have adequate statistical power, and by examining the effects of psychedelics across different menstrual cycle stages and developmental stages in female participants, we can ensure a more comprehensive understanding of psychedelic efficacy. Additionally, using standardized behavioral assays with known estrous effects can aid in data interpretation. This approach paves the way for more tailored and effective treatments for psychiatric conditions across sexes.

Abbreviations

5-HT

5-hydroxytryptamine

5-HT2AR

5-HT2A receptor subtype

AN

anorexia nervosa

AUD

alcohol use disorder

BDNF

brain-derived neurotrophic factor

CeA

central nucleus of the amygdala

DOI

2,5-dimethoxy-4-iodoamphetamine

E2

estradiol

ED

eating disorder

ER

estrogen receptor

FC

functional connectivity

HTR

head-twitch response

LSD

lysergic acid diethylamide

MDD

major depressive disorder

mTOR

mechanistic target of rapamycin

OCD

obsessive-compulsive disorder

SABV

sex as a biological variable

SERT

serotonin reuptake transporter gene

SSRI

selective serotonin reuptake inhibitor

SUD

substance use disorder

TPH

tryptophan hydroxylase

Funding

C.J.F. and Z.B.A. are supported by the National Health and Medical Research Council (NMHRC) of Australia (GNT2011334 to C.J.F.; GNT2013243 to Z.B.A.).

Disclosures

C.J.F. sits on the scientific advisory board for Octarine Bio, Copenhagen, Denmark. Z.B.A. is Editor in Chief of Endocrinology. The other authors have nothing to disclose.

Data Availability

Data sharing is not applicable to this article as no new data were created or analyzed in this study.