Sex-Specific Pharmacotherapy for Migraine: A Narrative Review

Sex differences in expression of CGRP family of receptors and ligands in the rat trigeminal system

BACKGROUND

Calcitonin gene-related peptide (CGRP) is part of the calcitonin peptide family, which includes calcitonin (CT), amylin (AMY), and adrenomedullin (ADM). CGRP and its receptor are highly present in the trigeminovascular system (TVS). Recent research suggests that other members of the calcitonin family could be feasible therapeutic targets in the treatment of migraine. The present study aims to elucidate the distribution of ADM, AMY, CT, and their receptors in the rat TVS, and to explore potential sex differences in their expression.

METHODS

Trigeminal ganglia (TG) were dissected from male and female adult rats. Protein and gene expression were assessed through immunohistochemistry and RT-qPCR. Additionally, the dura mater was isolated for further investigation of protein expression and fiber localization using immunohistochemistry.

RESULTS

Quantitative gene expression analysis revealed the presence of all genes in male and female TGs, except for calcitonin receptor (CTR). Notably, CGRP mRNA levels in TG were several folds higher than those of other genes. The receptor activity-modifying protein-1 (RAMP1) mRNA levels were significantly higher in female compared to male. No AMY or CT immunoreactivity was observed in the TVS. In contrast, immunoreactivity for ADM, CGRP, RAMP1, CTR, and calcitonin-like receptor (CLR) were observed in the cytoplasm of TG neurons. Immunoreactive Aδ-fibers storing RAMP1, ADM and CLR were also identified. RAMP2 and RAMP3 were expressed in nucleus of TG neurons and in satellite glial cells. Furthermore, RAMP1 and CLR were co-localized with CASPR in the nodes of Ranvier located in Aδ-fibers.

CONCLUSIONS

This study provides valuable insights into the distribution of the CGRP family of peptides and their receptors in the TVS. CGRP mRNA levels in the TG were markedly higher than those of other genes, demonstrating the key role of CGRP. The co-localization of CLR and RAMP1 on Aδ-fibers with CASPR suggests a potential role for this receptor in modulating trigeminal nerve function and neuronal excitability, with implications for migraine pathophysiology. Additionally, RAMP1 mRNA levels were significantly higher in female TG compared to males, indicating sex-specific differences in gene expression. These findings underscore the need for further research into the functional significance of gender-related variations.

Migraine treatment: quo vadis? Real-world data study (2015-2022) in Spain

BACKGROUND

Migraine is a leading cause of disability, estimated to affect one-in-ten people in Spain. This study aimed to describe the management of migraine in Spain and identify improvement areas.

METHODS

Non-interventional, retrospective, cross-sectional cohort study conducted using an electronic medical records database covering visits to public healthcare providers for 3% of the Spanish population. Patients with a migraine diagnosis (ICD-9 346) between 01/2015 and 04/2022 were included, as well as their demographic and clinical characteristics, prescribed migraine treatments and the specialty of the prescribing physicians.

RESULTS

The database included 61,204 patients diagnosed with migraine. A migraine treatment had been prescribed to 50.6% of patients over the last 24 months (only acute to 69.5%, both acute and preventive to 24.2%, and only preventive to 6.3%). The most frequently prescribed treatments were NSAIDs (56.3%), triptans (44.1%) and analgesics (28.9%). Antidepressants were the most common preventive treatment (prescribed to 17.9% of all treated patients and 58.7% of those treated with a preventive medication), and anti-CGRP monoclonal antibodies the least prescribed (1.7%; 5.7%). In 13.4% of cases, preventive medications were the first treatment: alone in 5.8% of cases and together with an acute medication in 7.6%. A fifth of patients who were initially prescribed with only acute treatment were later prescribed a preventive medication (20.7%). On average, it took 29.4 months for this change to occur. Two-thirds of patients started their preventive treatment in primary care (64.2%). The percentage of patients treated by a neurologist increased with the number of received preventive medications. However, 28.8% of patients who had already been prescribed five or more distinct preventive treatments were not treated by a neurologist. Migraine patients had between 1.2- and 2.2-times higher prevalence of comorbidities than the general population, age-gender adjusted.

CONCLUSIONS

Our study emphasizes the need for improved management of migraine in Spain to reduce the risk of chronification and improve patient outcomes. More training and coordination across healthcare professionals is necessary to recognize and address risk factors for migraine progression, including multiple associated comorbidities and several lines of treatment, and to provide personalized treatment plans that address the complex nature of the condition.

Central generators of migraine and autonomic cephalalgias as targets for personalized pain management: Translational links

BACKGROUND AND OBJECTIVE

Migraine oscillates between different states in association with internal homeostatic functions and biological rhythms that become more easily dysregulated in genetically susceptible individuals. Clinical and pre-clinical data on migraine pathophysiology support a primary role of the central nervous system (CNS) through 'dysexcitability' of certain brain networks, and a critical contribution of the peripheral sensory and autonomic signalling from the intracranial meningeal innervation. This review focuses on the most relevant back and forward translational studies devoted to the assessment of CNS dysfunctions involved in primary headaches and discusses the role they play in rendering the brain susceptible to headache states.

METHODS AND RESULTS

We collected a body of scientific literature from human and animal investigations that provide a compelling perspective on the anatomical and functional underpinnings of the CNS in migraine and trigeminal autonomic cephalalgias. We focus on medullary, hypothalamic and corticofugal modulation mechanisms that represent strategic neural substrates for elucidating the links between trigeminovascular maladaptive states, migraine triggering and the temporal phenotype of the disease.

CONCLUSION

It is argued that a better understanding of homeostatic dysfunctional states appears fundamental and may benefit the development of personalized therapeutic approaches for improving clinical outcomes in primary headache disorders.

SIGNIFICANCE

This review focuses on the most relevant back and forward translational studies showing the crucial role of top-down brain modulation in triggering and maintaining primary headache states and how these central dysfunctions may interact with personalized pain management strategies.

Gender bias in clinical trials of biological agents for migraine: A systematic review

Migraine is considered one of the most disabling diseases. Currently, there are few studies on clinical migraine treatment based on sex-related differences, despite the important role of sex in migraine. Our aim was to evaluate gender bias in published clinical trials on monoclonal antibodies (erenumab, galcanezumab, fremanezumab and eptinezumab). We performed a systematic review of controlled clinical trials of erenumab, galcanezumab, fremanezumab and eptinezumab, searching the PubMed/MEDLINE database for articles published before December 2021. The search identified 760 articles, 25 of which met the inclusion criteria. Of all the patients included in these trials, 85.1% were women. Only one study had female lead authors. Two of the 25 studies included a sex-based analysis of the primary endpoint. None of the articles discussed the results separately for men and for women. The proportion of men recruited in trials is scarce and more studies are needed to guarantee the safety and tolerability of monoclonal antibodies used in male migraine. As observed in our study, despite the high number of women recruited, only 2 studies analysed the results separately by sex. Thus, a potential risk of gender bias was found in these clinical trials.

Relation and Treatment Approach of Migraine in Pregnancy and Breastfeeding

Headaches are one of the most frequent reasons people visit the neurology department. In 2019, headache issues ranked as the 14th most common cause of disability-adjusted life years globally. According to the International Headache Society, migraine is a particular type of headache that is unilateral, frequently throbbing, associated with vertigo, and sensitive to light, sound, and head movement. A migraine has four stages: premonitory, aura, headache, and postdrome. Migraine is the type of discomfort that most frequently complicates the pregnancy. A migraine is more common in women than in men. Migraines are influenced by increased levels of estrogen during pregnancy and a sharp decrease in those levels during puerperium. Untreated migraine can result in premature labor, hypertension, and low birth weight babies. Menstrual-related events occurred more frequently in migraine sufferers than in non-migraine sufferers. We have explained the relation of sex hormones that trigger migraine. We have also reviewed the therapeutic approach, such as pharmacological and non-pharmacological approaches, for migraine in pregnancy and breastfeeding. A migraine episode during menstruation was slightly more severe and complicated than a headache that wasn't a migraine. Breastfeeding is not prohibited by migraines. The steady estrogen levels brought on by lactating women's lack of menstruation may function as a protective factor. In addition to any required drug therapy, nonpharmacological techniques should always be used as the first line of treatment. Preconception counseling is an essential part of providing headache patients with safe therapy during pregnancy. Supplemental estrogen should not be used by any women who have an aura or who are 35 years of age or older because there is inadequate proof to support any long-term adverse effects. Paracetamol is advised for use in acute therapy during pregnancy. Mothers who used acetaminophen during pregnancy are more likely to have children with hyperkinetic disorders and characteristics resembling attention-deficit/hyperactivity disorder. Menstrual migraine can be treated in a variety of ways, including acute therapy, non-pharmacological therapy, and preconception counseling. Similar tactics are used to treat migraines during pregnancy, but it's important to take the medications' safety rating into account. Migraines and menstrual problems go together. A safeguarding element is a constant estrogen level during pregnancy and breastfeeding. The preferred method of treatment for migraine is non-pharmacological therapy, followed by prenatal counseling. Sumatriptan and acetaminophen are both effective treatments for transient migraine attacks that occur during pregnancy or breastfeeding.

Rimegepant for the treatment of migraine

Migraine is a common form of primary headache, affecting up to 1 in every 6 Americans. The pathophysiology is an intricate interplay of genetic factors and environmental influence and is still being elucidated in ongoing studies. The trigeminovascular system is now known to have a significant role in the initiation of migraines, including the release of pain mediators such as CGRP and substance P. Traditional treatment of migraine is usually divided into acute and preventive treatment. Acute therapy includes non-specific therapy, such as NSAIDs and other analgesics, which may provide relief in mild to moderate migraines. 5-HT1 agonists may provide relief in severe migraine, but are not universally effective and carry a significant side-effect profile with frequent redosing requirement. Prophylactic therapy may reduce the occurrence of acute migraine attacks in selected patients, but does not completely eliminate it. More recently, CGRP antagonism has been studied and shown to be effective in both abortion and prevention of migraine. Novel medications, targeting CGRP, divide into CGRP antibodies and receptor antagonists (gepants). Rimegepant, a second-generation gepant, has shown efficacy in several clinical trials in treating acute migraine. Ongoing trials are also evaluating its role in migraine prophylaxis, and results are promising. It is also generally safer for use than existing options, does not appear to increase the chance of developing chronic migraines, and carries a very tolerable side effects profile. It is a part of a growing arsenal in migraine treatment, and may present the silver bullet for treatment of this disease.

Headache, eating disorders, PTSD, and comorbidity: implications for assessment and treatment

PURPOSE

Severe headaches (HAs) have been associated with eating disorders (ED) as well as with trauma, posttraumatic stress disorder (PTSD), major depression and anxiety. However, data addressing all of these factors in the same subjects are limited.

METHODS

In a large sample of patients (n = 1461, 93% female) admitted to residential treatment (RT) for an ED, we assessed within 48-72 h of admission subjective reports of frequent HAs and their associations with severity of ED, PTSD, major depressive and state-trait anxiety symptoms, as well as quality of life measures. HA ratings were significantly correlated to the number of lifetime trauma types as well as to symptoms of PTSD, major depression, and state-trait anxiety.

RESULTS

Results indicated that 39% of patients endorsed that frequent HAs occurred "often" or "always" (HA+) in association with their eating or weight issues. This HA-positive (HA+) group had statistically significant higher numbers of lifetime trauma types, higher scores on measures of ED, PTSD, major depressive, and state-trait anxiety symptoms, and worse quality of life measures (p ≤ 0.001) in comparison to the HA-negative (HA-) group, who endorsed that frequent HAs occurred "never," "rarely," or "sometimes" in association with their eating or weight issues. The HA + group also had a significantly higher rate of a provisional PTSD diagnosis (64%) than the HA- group (35%) (p ≤ .001). Following comprehensive RT, HA frequency significantly improved (p ≤ .001).

CONCLUSION

These findings have important implications for the assessment and treatment of HAs in the context of ED, PTSD and related psychiatric comorbidities, especially at higher levels of care. In addition, the importance of identifying traumatic histories and treating comorbid PTSD and related psychopathology in individuals presenting with severe HAs is emphasized.

LEVEL OF EVIDENCE

III Evidence obtained from well-designed cohort or case-control analytic studies.

Unpredictable Sound Stress Model Causes Migraine-Like Behaviors in Mice With Sexual Dimorphism

Migraine represents one of the major causes of disability worldwide and is more prevalent in women; it is also related to anxiety symptoms. Stress, such as sound stress, is a frequently reported trigger in migraine patients, but the underlying mechanisms are not fully understood. However, it is known that patients with migraine have higher levels of plasma inflammatory cytokines and calcitonin gene-related peptide (CGRP). Stress mediated by unpredictable sound is already used as a model of painful sensitization, but migraine-like behaviors and sexual dimorphism have not yet been evaluated. This study characterized nociception and anxiety-related symptoms after the induction of sound stress in mice. C57BL/6 mice (20-30 g) were exposed to unpredictable sound stress for 3 days, nonconsecutive days. We observed enhanced plasma corticosterone levels on day 1 after stress induction. First, 7 days after the last stress session, mice developed hind paw and periorbital mechanical allodynia, grimacing pain behavior, anxiety-like symptoms, and reduced exploratory behavior. The nociceptive and behavioral alterations detected in this model were mostly shown in female stressed mice at day 7 post-stress. In addition, on day 7 post-stress nociception, these behaviors were consistently abolished by the CGRP receptor antagonist olcegepant (BIBN4096BS, 100 mg/kg by intraperitoneal route) in female and male stressed mice. We also demonstrated an increase in interleukine-6 (IL-6), tumor necrosis factor (TNF-α), and CGRP levels in stressed mice plasma, with female mice showing higher levels compared to male mice. This stress paradigm allows further preclinical investigation of mechanisms contributing to migraine-inducing pain.

B vitamins and their combination could reduce migraine headaches: A randomized double-blind controlled trial

Background: The B vitamins can potentially help prevent migraine. This study was designed to examine the effects of supplementation with thiamine (B1), pyridoxine (B6), cobalamin (B12), folic acid (B9), and a combination of these vitamins on women with episodic migraine (EM). Methods: This study was a double-blind, placebo-controlled, randomized, clinical trial conducted on 120 women with EM. The participants were divided into the 6 groups of B1 (n = 20), B6 (n = 20), B12 (n = 20), B9 (n = 20), vitamin B complex (n = 20), and placebo (n = 20). Subjects received 1 capsule daily for 12 weeks. As part of the baseline and post-intervention phases, paper-based headache diaries were used to record the number of abortive drugs consumed and the frequency of headache attacks, and the Migraine Disability Assessment Questionnaire (MIDAS) was used to assess migraine disability. Results: A 16-week study on women with EM revealed that the mean changes in the frequency of headache attacks decreased significantly in all vitamin groups in comparison with the placebo group (P < 0.001). In contrast to the placebo, there was also a significant improvement in the migraine disability score in each vitamin group (P < 0.001). The 12-week supplementation with vitamins B9, B1, B6, B12, and B complex also brought on a significant decrease in the use of abortive drugs compared to the placebo group (P = 0.032). Conclusion: The results of this study showed that B1, B6, B12, and B9, and a combination of these vitamins could be effective as an adjuvant in treatment and prophylaxis of EM. Further large trials with long-term follow-ups will be required to confirm our results.

Neurogenic Inflammation: The Participant in Migraine and Recent Advancements in Translational Research

Migraine is a primary headache disorder characterized by a unilateral, throbbing, pulsing headache, which lasts for hours to days, and the pain can interfere with daily activities. It exhibits various symptoms, such as nausea, vomiting, sensitivity to light, sound, and odors, and physical activity consistently contributes to worsening pain. Despite the intensive research, little is still known about the pathomechanism of migraine. It is widely accepted that migraine involves activation and sensitization of the trigeminovascular system. It leads to the release of several pro-inflammatory neuropeptides and neurotransmitters and causes a cascade of inflammatory tissue responses, including vasodilation, plasma extravasation secondary to capillary leakage, edema, and mast cell degranulation. Convincing evidence obtained in rodent models suggests that neurogenic inflammation is assumed to contribute to the development of a migraine attack. Chemical stimulation of the dura mater triggers activation and sensitization of the trigeminal system and causes numerous molecular and behavioral changes; therefore, this is a relevant animal model of acute migraine. This narrative review discusses the emerging evidence supporting the involvement of neurogenic inflammation and neuropeptides in the pathophysiology of migraine, presenting the most recent advances in preclinical research and the novel therapeutic approaches to the disease.

Impact of age and sex on the efficacy of fremanezumab in patients with difficult-to-treat migraine: results of the randomized, placebo-controlled, phase 3b FOCUS study

Background

Migraine prevalence is age and sex dependent, predominating in women in early and middle adulthood; however, migraine also represents a substantial burden for men and adults of all ages. Thus, understanding this burden and the efficacy of migraine preventive medications in both sexes and across age groups is critical. The randomized, placebo-controlled, double-blind, phase 3b FOCUS study demonstrated the safety and efficacy of fremanezumab, a fully humanized monoclonal antibody (IgG2∆a) that selectively targets calcitonin gene-related peptide as a migraine preventive treatment for individuals with migraine and prior inadequate response to 2 to 4 migraine preventive medication classes. Here, we assessed the efficacy of fremanezumab in participants from FOCUS subgrouped by age (18–45 years and > 45 years) and sex.

Methods

In the FOCUS study, eligible participants were randomized (1:1:1) to 12 weeks of double-blind treatment with quarterly fremanezumab, monthly fremanezumab, or matched monthly placebo. In this post hoc analysis, we evaluated changes from baseline in monthly migraine days (primary endpoint of FOCUS) and other secondary and exploratory efficacy outcomes in prespecified age (18–45 and > 45 years) and sex subgroups.

Results

The modified intention-to-treat population (received ≥ 1 dose of study drug and had ≥ 10 days of postbaseline efficacy assessments for the primary endpoint) totaled 837 participants (18–45 years, n = 373; > 45 years, n = 464; male, n = 138; female, n = 699). Consistent reductions in monthly average number of migraine days during 12 weeks were observed, regardless of age (18–45 years: quarterly fremanezumab, − 4.1 days; monthly fremanezumab, − 4.7 days; placebo, − 0.9 days; P < 0.001; > 45 years: quarterly fremanezumab, − 3.6 days; monthly fremanezumab, − 3.7 days; placebo, − 0.3 days; P < 0.001) and sex (male: quarterly fremanezumab, − 4.1 days; monthly fremanezumab, − 4.6 days; placebo, − 0.3 days; P < 0.001; female: quarterly fremanezumab, − 3.6 days; monthly fremanezumab, − 3.9 days; placebo, − 0.6 days; P < 0.001). Fremanezumab also reduced monthly headache days of at least moderate severity, monthly days of acute medication use, and improved Migraine Disability Assessment scores across subgroups.

Conclusions

These results demonstrate the efficacy of fremanezumab in patients with difficult-to-treat migraine for reducing migraine and headache days, acute medication use, and disability, regardless of age or sex.

Trial registration

ClinicalTrials.gov Identifier NCT03308968 (FOCUS), registered October 13, 2017.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-021-01336-1.

A link between migraine and prolactin: the way forward

Migraine is an incapacitating neurological disorder that predominantly affects women. Sex and other hormones (e.g., oxytocin, and prolactin) may play a role in sexual dimorphic features of migraine. Initially, prolactin was recognized for its modulatory action in milk production and secretion; later, its roles in the regulation of the endocrine, immune and nervous systems were discovered. Higher prolactin levels in individuals with migraine were found in earlier studies, with a female sex-dominant trend. Studies that are more recent have identified that the expression of prolactin receptor in response to neuronal excitability and stress depends on sex with a dominant role in females. These findings have opened up potentials for explanation of sex-related pathophysiology of migraine, but have left some unanswered questions. This focused review examines the past and present of the link between prolactin and migraine, and presents open questions and directions for future experimental and clinical efforts.

Sex hormones (e.g., estrogen and progesterone) have been investigated to explain the sex-related manifestation of migraine, which is predominant in females. Prolactin is known for promoting lactation, but accumulating evidence supports that it can promote pain in females. An increasing number of studies have shown that the expression of a prolactin receptor in female nociceptors and their responses to external stimuli such as stress are different, which can help explain the female sex-dominant feature of migraine. In this focused review, the current knowledge is presented and the directions where prolactin research in migraine may evolve are proposed. The ultimate goal is to shape an overview toward considering sex-based treatments for migraine with highlighting the role of prolactin.

Hormonal influences in migraine - interactions of oestrogen, oxytocin and CGRP

Migraine is ranked as the second highest cause of disability worldwide and the first among women aged 15-49 years. Overall, the incidence of migraine is threefold higher among women than men, though the frequency and severity of attacks varies during puberty, the menstrual cycle, pregnancy, the postpartum period and menopause. Reproductive hormones are clearly a key influence in the susceptibility of women to migraine. A fall in plasma oestrogen levels can trigger attacks of migraine without aura, whereas higher oestrogen levels seem to be protective. The basis of these effects is unknown. In this Review, we discuss what is known about sex hormones and their receptors in migraine-related areas in the CNS and the peripheral trigeminovascular pathway. We consider the actions of oestrogen via its multiple receptor subtypes and the involvement of oxytocin, which has been shown to prevent migraine attacks. We also discuss possible interactions of these hormones with the calcitonin gene-related peptide (CGRP) system in light of the success of anti-CGRP treatments. We propose a simple model to explain the hormone withdrawal trigger in menstrual migraine, which could provide a foundation for improved management and therapy for hormone-related migraine in women.

Gender Specific Differences in Disease Susceptibility: The Role of Epigenetics

Many complex traits or diseases, such as infectious and autoimmune diseases, cancer, xenobiotics exposure, neurodevelopmental and neurodegenerative diseases, as well as the outcome of vaccination, show a differential susceptibility between males and females. In general, the female immune system responds more efficiently to pathogens. However, this can lead to over-reactive immune responses, which may explain the higher presence of autoimmune diseases in women, but also potentially the more adverse effects of vaccination in females compared with in males. Many clinical and epidemiological studies reported, for the SARS-CoV-2 infection, a gender-biased differential response; however, the majority of reports dealt with a comparable morbidity, with males, however, showing higher COVID-19 adverse outcomes. Although gender differences in immune responses have been studied predominantly within the context of sex hormone effects, some other mechanisms have been invoked: cellular mosaicism, skewed X chromosome inactivation, genes escaping X chromosome inactivation, and miRNAs encoded on the X chromosome. The hormonal hypothesis as well as other mechanisms will be examined and discussed in the light of the most recent epigenetic findings in the field, as the concept that epigenetics is the unifying mechanism in explaining gender-specific differences is increasingly emerging.

Clinical Manifestations of Overactive Bladder With Migraine as a Comorbidity: A Prospective Cross-Sectional Study

Purpose

The aim of this study was to investigate the clinical manifestations of overactive bladder (OAB) with migraine as a comorbidity and to shed light on possible new treatment strategies.

Methods

This study included patients aged 18 years and older who were admitted to urology and neurology outpatient clinics between March 1, 2019 and March 1, 2020 for OAB and migraine. The study questionnaire contained 3 sections: (1) questions on demographic characteristics, (2) a migraine ID test, and (3) the Overactive Bladder Inquiry Form - V8 (OAB-V8) form.

Results

A total of 265 patients participated in the study. The average age of the participants was 39.75±11.93 years. The patients were divided into 3 groups according to the coexistence of OAB with migraine: group 1, OAB(+)/migraine(+); group 2, OAB(+)/migraine(-); and group 3, OAB(-)/migraine(+). The mean OAB-V8 score was 22.82 ±8.15 in group 1 and 25.64±7.49 in group 2. The mean OAB-V8 score of OAB patients with migraine as a comorbidity was statistically significantly lower than that of OAB patients without migraine (P=0.015). The median visual analogue scale (VAS) score was 7.11 (range, 2–10) in group 1 and 5.95 (range, 2–10) in group 3. This finding indicates that in patients with migraine, having OAB was associated with significantly higher VAS scores (P<0.001).

Conclusions

OAB and migraine may be comorbid conditions coexisting in a single patient. This comorbidity may lead to a lower perception of OAB symptoms in OAB patients or, conversely, to a higher perception of migraine pain. Further studies are needed to elucidate how treatments for each of these diseases can affect the other disease.

Sex and the serotonergic underpinnings of depression and migraine

Most psychiatric disorders demonstrate sex differences in their prevalence and symptomatology, and in their response to treatment. These differences are particularly pronounced in mood disorders. Differences in sex hormone levels are among the most overt distinctions between males and females and are thus an intuitive underpinning for these clinical observations. In fact, treatment with estrogen and testosterone was shown to exert antidepressant effects, which underscores this link. Changes to monoaminergic signaling in general, and serotonergic transmission in particular, are understood as central components of depressive pathophysiology. Thus, modulation of the serotonin system may serve as a mechanism via which sex hormones exert their clinical effects in mental health disorders. Over the past 20 years, various experimental approaches have been applied to identify modes of influence of sex and sex hormones on the serotonin system. This chapter provides an overview of different molecular components of the serotonin system, followed by a review of studies performed in animals and in humans with the purpose of elucidating sex hormone effects. Particular emphasis will be placed on studies performed with positron emission tomography, a method that allows for human in vivo molecular imaging and, therefore, assessment of effects in a clinically representative context. The studies addressed in this chapter provide a wealth of information on the interaction between sex, sex hormones, and serotonin in the brain. In general, they offer evidence for the concept that the influence of sex hormones on various components of the serotonin system may serve as an underpinning for the clinical effects these hormones demonstrate.