Many physiologic systems and processes have been implicated as contributing to the etiopathogenesis of depression. These include neurotransmission, neuroplasticity, stress axis, immune function, and neural and genetic network regulation. As described below, sex-related differences as well as reproductive steroid-related modulation have been observed in each of these processes. (The existence of reproductive steroid regulatory effects provides a basis for inferring sex-dependent differential function of these systems, but, as noted above, differences at the molecular level may converge to result in similar physiologic endpoints.) Several examples follow, with greater attention paid to dysregulation of immune function, stress reactivity, and neural network function.

Neurotransmission

As described above, the effects of reproductive steroids on neurotransmission are profound, affecting all levels of function including neurotransmitter synthesis and metabolism, receptor synthesis and trafficking, and signal transduction. Not surprisingly, therefore, there are many reported sex differences, many of which are brain region specific, in the concentrations, receptors, and metabolites of classical neurotransmitters as well as in their elicited intracellular signals and cellular actions. These are reviewed elsewhere (see refs. [86, 92, 110, 111, 114, 115]).

Neuroplasticity

Alterations in neuroplasticity, including neurogenesis, cell death, and synaptic remodeling, are some of the fundamental processes that underlie the development of sex differences in the brain [194]. Sex steroids differentially regulate many of the molecules involved in neuroplasticity (e.g., neurotrophin secretion), as well as several of the physiologic systems (e.g., glutamatergic and GABA-ergic) [112, 236] regulating the opening and closing of critical developmental windows [237]. Thus, exposures to sex steroids could regulate sex-specific “opportunities” for the effects of physiologic events (e.g., puberty) or adversity to differentially impact developmental (re)programming and the instantiation of behavioral risk over the life cycle. Sex differences in the mechanisms of neuroplasticity have been reported, particularly in the hippocampus, a brain region implicated in affective dysregulation [238]. In knock-out mice, calcium/calmodulin kinase kinases appear to play a more significant role in hippocampal neuroplasticity in males compared to females [239], whereas females but not males show synapse induction within the hippocampus by E2 (although testosterone will induce synapse formation in males (in vivo) [240]). Similarly, the pattern of hippocampal synaptic remodeling of CA3 dendrites after chronic stress exposure is observed in males but not females [241] (reviewed in ref. [113]). The relevance of these differences in hippocampal neuroplasticity to sex differences in human brain function (or disease) remains to be clarified. Additionally, several sex differences in neuroplasticity after administration of ketamine (an N-methyl-d-aspartate (NMDA) receptor antagonist used for treating depression [242]) have been reported in socially isolated rodents [243]. For example, ketamine reversed the decreased spine density in the medial prefrontal cortex in males but not females [244]. These differences also likely reflect differences in the impact of social isolation on male and female mice [245, 246]. Recently, female mice were observed to have threefold increased levels of hydroxynorketamine, the ketamine metabolite with rapid antidepressant-like actions, in the absence of NMDA receptor inhibition [247]. Thus, although a recent report from human studies did not identify sex differences in the antidepressant effects of ketamine in treatment-resistant depression [248], it is possible that sex differences in the sensitivity to ketamine could emerge in larger samples.

There is considerable evidence that reproductive steroids modulate neuroplastic processes implicated in depression and/or the antidepressant response. E2, for example, does the following: acts like antidepressants (and opposite to stress) in stimulating BDNF [249]; increases activity of the transcription factor cAMP response element-binding protein (CREB) [250] and trkA (neurotrophic tyrosine kinase receptor type 1) [251, 252]; and decreases glycogen synthase kinase-3β in rat brain [253], the same direction of effects as seen with mood stabilizers. Interestingly, sex differences are observed in the facilitative effects of E2 on neuroplasticity. Although the effect on synaptic activity (i.e., potentiation of glutamatergic synapses in CA1 hippocampal slices) is similar, the mechanism involves ER-β (and possibly GPER1) in females vs. ER-α in males [222].

Neuroprotective effects of E2 and progesterone (or its neurosteroid metabolites) have also been described in neurons grown in serum-free media or those exposed to glutamate, amyloid-β, hydrogen peroxide, ischemia, or glucose deprivation [254,255,256,257,258] (see also ref. [259]). Some of these effects appear to lack stereospecificity (i.e., are not classical steroid-receptor mediated) and may be attributable to the antioxidant properties of E2 or the GABA A -modulating effects of allopregnanolone, although more recent data support steroid receptor-mediated mechanisms of action. Gonadal steroids may also modulate cell survival through effects on cell survival proteins (e.g., Bcl-2, BAX), signaling pathways (e.g., MAPK, Akt), intracellular calcium regulation, metabolism of amyloid precursor protein and Aβ, or through enhancing mitochondrial respiratory efficiency [259,260,261,262]. Through increased oxidative capacity and efficiency of neuronal mitochondria, E2 promotes neuronal bioenergetics and protects neurons against multiple toxins, including free radicals, excitotoxins, Aβ, and ischemia [260]. Damage from oxidative stress to mitochondria promotes apoptosis and cell death, and both estrogen receptor-dependent and -independent neuroprotection at the level of the mitochondria have been described. For example, E2 bound to ER-β can be transported into the mitochondria, where it binds an ERE in mitochondrial DNA [188, 189, 263] and produces a range of anti-apoptotic proteins that maintain the integrity of the mitochondrial membranes. Thus, in addition to its other neurotrophic actions [263], E2 could improve mitochondrial respiratory efficiency by directly inducing transcriptional activity in mitochondrial DNA and prevent the oxygen free radicals that are believed to adversely affect mitochondrial energetics in depression.

Neural and gene network function

Gene networks

A study of gene co-expression networks showed marked differences between patients with MDD and controls, but strikingly there was little overlap in the altered transcriptional network connectivity patterns in males and females with depression or in mice subjected to chronic variable stress [264]. Of note in this study, a hub gene—Dusp6—was downregulated in a sex-specific fashion in both depressed women and stressed mice, regulated cell signaling and ventromedial PFC pyramidal cell excitation (only in females), and, when downregulated, increased behavioral sensitivity to stressors; i.e., identical manipulations of the same gene led to major differences in both the physiological and behavioral effects in males and females. The sex-specific transcriptional signatures in depression were recently replicated by Seney et al. [265], who observed that of the 700 to almost 900 genes differentially expressed in cortico-limbic regions in men and women with MDD compared with controls, only 73 genes were differentially expressed in both men and women, and 52 of these changed in the opposite direction. Additionally, postmortem studies of men and women with MDD have reported a sex difference in the expression within the dorsolateral prefrontal cortex of multiple glutamatergic genes, with increased expression in women [266]. Finally, several clock genes have been reported to differ in a sex-specific manner, possibly in keeping with reports that women have an overall phase advance in several measures of circadian rhythmicity compared with men [267,268,269].

Neural networks

Disturbances of amygdalar activity or connectivity have been implicated in affective disorders [270], and sex differences have been described in amygdala activation patterns [271] and connectivity [272]. Of note, the amygdala is differentially activated in men and women as a function of the valence or nature of the affective stimulus, positive or sexual in men and negative in women [273].

Sex differences have been observed in resting state functional connectivity, with several studies finding increased connectivity within statistically defined modules or sensorimotor resting networks (i.e., increased intra-network/module connectivity) in women and increased inter-network or cross-module connectivity in men [234, 274], with the caveat that studies also have reported increased intra- and cross-modular connectivity in girls compared to boys (Reding et al., unpublished observations) as well as other sex-specific patterns of connectivity in boys and girls [275,276,277]. As emphasized by Mak et al. [277], the divergence in some of these functional connectivity findings reflect differing analytic methods (e.g., independent component analyses vs. seed-based vs. data-driven analyses), potential presence of negative affective symptoms on the day of scanning, and differing ages of participants (e.g., prepuberty vs. adults). Nonetheless, abnormalities in resting state functional connectivity within the default mode network (DMN) have been observed in depressed men and women [157, 158, 278]. However, few if any studies report sex differences in DMN functional connectivity in depression. Recent studies do suggest an association between depressive rumination and functional connectivity between the DMN and the subgenual prefrontal cortex (sgPFC) [157]. Since studies also suggest that women experience a greater amount of rumination during depression than men [279], one could imagine that a sex difference specifically in DMN functional connectivity with the sgPFC may be detected. Alternatively, if confirmed, the presence of sex differences in DMN network activity in asymptomatic men and women, and the absence of comparable differences in depression, could inform our understanding of depressive illness.

A sex difference has been reported in an emotional/arousal network, with men showing greater activation (fMRI blood-oxygen-level dependent (BOLD)) of the anterior cingulate gyrus, OFC, hippocampus, and mPFC when watching negatively valenced/high arousal pictures compared with women during mid-cycle of the menstrual cycle [271]. Using a similar paradigm, these investigators previously also demonstrated that activation patterns in many of these brain regions in women decreased during mid-cycle compared with the early follicular phase, suggesting that higher estradiol levels at mid-cycle attenuated the activity of this network [148]. Thus, sex differences in network level function may also reflect the regulatory effects of sex steroids. This observation is lent further support by the recent discovery of an estradiol-regulated reward circuit in mice [280], again suggesting the contribution of activational effects of sex steroids to differential network function in depression.

Immune function

Multiple lines of evidence suggest that immune dysfunction contributes to the risk for depression. Raison and Miller [281] have proposed the pathogen:host defense model, which suggests that immune activation and stress perception have “co-evolved” to generate sickness behavior (protective in intent) in response to environmental threats and challenges. Additionally, multiple studies identify elevated immune activation markers and cytokine levels (e.g., C-reactive protein, interleukin (IL)-6, IL-1B, and tumor necrosis factor-α (TNF-α)) in patients with major depression compared with controls [282, 283]. Multiple sex differences have been described for immune function, stress response (see below), and the interaction of stress and immune function in depression.

Sex differences in immune function or effects include the following. (1) In response to immune activation, females experience more adverse behavioral effects, including increased immobility on the forced swim test and decreased sucrose preference in rodents [284,285,286] and increased depression and social disconnection in women [287, 288]. (2) In the immune response itself, in humans some immune cells increased equally in males and females after stress, some to a greater extent in males, some more in females regardless of menstrual cycle phase, and some only in females on oral contraceptives, findings again consistent with both programming and activational sources of sex differences [289]. Further, although lipopolysaccharide-stimulated microglial IL-1B expression in vitro is increased in neonatal males compared with females, it is suppressed by E2 in males and increased by E2 in females, suggesting that not only is the sex difference hormonally responsive, but the effect of the same hormone is opposite in males and females [290]. (3) Sex differences exist in the susceptibility to neuroimmune-related disorders. Autoimmune disorders (including multiple sclerosis (MS), lupus, rheumatoid arthritis, and thyroid disease) show a dramatically increased prevalence (2–9-fold) in women [291], consistent with the increased susceptibility to experimental autoimmune encephalitis (EAE), an animal model of MS, in female rodents [292]. Notably, in the EAE model, the same trigger (myelin basic protein) leads to decreased lymph node immune cells, decreased reactive cells, and decreased cytokines, as well as increased spleen-derived “anti-inflammatory” cytokines in males [292, 293]. The same stimulus, then, produces both different immune responses and different delivery of immune signals to the brain. Despite the female predominance of EAE, E2 improves EAE severity in both males and females [294]. Indeed, while multiple studies support the immunomodulatory effect of E2, suggesting its suppression of the microglial “inflammatory” state and dose-dependent suppression of the synthesis of a range of cytokines (TNF-α, IL-1B, MCP2) [295], the role of E2 in the regulation of immune response is complex and likely both cell- and context-dependent [294]. Sex steroids are key regulators of immune cell phenotype and function, with demonstrated roles in the regulation of both adaptive and innate immunity. Androgens and estrogens have been shown to regulate immune cell proliferation, differentiation, and apoptosis, as well as cytokine and immunoglobulin production [296,297,298]. Of note, sex steroids are synthesized de novo in the brain (neurons, glia), and hence their immunomodulatory effects can occur locally, representing paracrine and autocrine rather than classically endocrine effects (reviewed in ref. [299].

Stress axis

The importance of antecedent stress, stress sensitivity, and stress axis dysregulation in affective illness is, at this point, axiomatic. Stress in relation to affective disorders can be viewed from three distinct perspectives—stress as an environmental stimulus, the stress axis as an outcome measure, and the stress axis as a mediator of change in physiology (e.g., neural network or immune function) or behavior. Sex differences in stress are reviewed elsewhere [300,301,302], but several deserve mention in relation to affective disorder. First, as noted above, sex may elicit different responses from the environment (i.e., women are subject to different, potentially stressful interactions because of their sex). Second, women are twice as likely to experience stress-related disorders (MDD, anxiety, PTSD, obesity (also eating disorders and most phobias)) and more likely to develop them after trauma, physical abuse, or maternal distress during infancy [303,304,305,306,307,308]. Prenatal stress results in earlier behavioral problems in boys, with girls showing stronger effects (and increased amygdalar volumes) later [309, 310]. Third, the hypothalamic–pituitary–adrenal (HPA) axis may respond differently as a function of sex and is regulated by reproductive steroids. Sex differences in measures of HPA axis activity take the form of differences in magnitude, effects in one sex but not the other, and opposite effects. For example, men have greater stimulation of adrenocorticotropic hormone (ACTH) and cortisol compared with women after the Trier Social Stress Test (TSST) [311], and both CRH- and exercise-stimulated ACTH and cortisol are greater in men than women even in the absence of differences in reproductive steroids (i.e., under GnRH-induced hypogonadism [312], although also see ref. [302]). Early trauma is positively associated with basal ACTH in women and negatively associated in men, while severe trauma is associated positively with ACTH response to CRH in men, but not women [313]. Of note, this literature is filled with ostensible inconsistencies, no doubt a function of the nature, duration, and timing (developmentally) of the stressors as well as the measures obtained.

Not surprisingly, sex differences have been reported in many components of the stress response, including differences in elicited changes in brain structure, non-HPA physiologic response (e.g., immune response noted above), and behavior. Chronic stress causes atrophy of dendrites in medial prefrontal cortex and hippocampus (CA3 pyramidal cells) only in male rodents [314, 315], whereas projections from medial prefrontal cortex to amygdala show (estrogen-dependent) increased spine density and dendritic expansion after chronic stress only in females [316]. These data are complemented by behavioral studies demonstrating that stressors like tail pinch increase associative learning (e.g., classically conditioned eyeblink response) in male rodents and produce learning deficits in females [317, 318]. Further, stress effects on learning are mediated by different brain regions and circuits in males and females (i.e., the mPFC, particularly in its connection to the amygdala, is critical to disrupted learning in females but not to enhanced learning in males, which involves activation of the BNST (not seen in females)) [319]. It should be noted that the sex differences in learning after acute stress are reversed for chronic stress, in which memory deficits are observed in males, but not females [236, 320]. As described above, the same repeated stress paradigm producing similar deficits in genetically identical male and female mice yields amazingly distinct, sex-dependent profiles of differentially expressed genes (about 20% overlap) compared with unstressed control animals [264]. Further, subchronic variable stress induces multiple depression-associated behavioral deficits (e.g., decreased sucrose preference, latency to eat in a novel environment) in female but not male mice, with, again, sex-dependent difference in the transcriptional response to stress in the nucleus accumbens [321]. A variety of findings converge in suggesting that women differentially process stressful stimuli: negative, arousing stimuli evoke faster and greater EEG responses in women [322], and emotion-evoking tasks produce greater activation of the locus coeruleus in women than men [323]. In parallel, the dendritic structure and post CRF receptor signaling in the locus coeruleus in rodents favor an exaggerated response to stress in females [324].

Effects of gonadal steroids

Many of the myriad sex differences in stress processing are attributable to differential exposure to reproductive steroids, which, with their metabolites, play a major role in modulation of the stress response. The CRF receptor has an ERE [325], and many components of the HPA axis vary across the estrus or menstrual cycle. Basal and stimulated ACTH and cortisol levels are higher in female rodents and during proestrus (when E2 and progesterone levels are high) [326,327,328]. These sex differences are eliminated by ovariectomy, as are many of the sex differences in brain region-specific structural and functional effects of stress noted above (e.g., the expansion of dendrites and spines in the mPFC–amygdala projections in females [316] or the sex-divergent effects of acute stress on classical eyeblink conditioning [319]). Of note, metabolites of reproductive steroids also regulate the response to stress. Thus, metabolites of progesterone (allopregnanolone) and testosterone (dihydrotestosterone) both dampen the response to stress in rodents, the former through acute effects on the GABA receptor and the latter through ER-β [329, 330].