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. Author manuscript; available in PMC: 2017 Jun 1.
Published in final edited form as: J Steroid Biochem Mol Biol. 2015 Dec 9;160:196–203. doi: 10.1016/j.jsbmb.2015.11.019

GABAergic regulation of the HPA and HPG axes and the impact of stress on reproductive function

Laverne Camille Melón 1, Jamie Maguire 1
PMCID: PMC4861672  NIHMSID: NIHMS746207  PMID: 26690789

Abstract

The hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes are regulated by GABAergic signaling at the level of corticotropin-releasing hormone (CRH) and gonadotropin-releasing hormone (GnRH) neurons, respectively. Under basal conditions, activity of CRH and GnRH neurons are controlled in part by both phasic and tonic GABAergic inhibition, mediated by synaptic and extrasynaptic GABAA receptors (GABAARs), respectively. For CRH neurons, this tonic GABAergic inhibition is mediated by extrasynaptic, δ subunit-containing GABAARs. Similarly, a THIP-sensitive tonic GABAergic current has been shown to regulate GnRH neurons, suggesting a role for δ subunit-containing GABAARs; however, this remains to be explicitly demonstrated. GABAARs incorporating the δ subunit confer neurosteroid sensitivity, suggesting a potential role for neurosteroid modulation in the regulation of the HPA and HPG axes. Thus, stress-derived neurosteroids may contribute to the impact of stress on reproductive function. Interestingly, excitatory actions of GABA have been demonstrated in both CRH neurons at the apex of control of the HPA axis and in GnRH neurons which mediate the HPG axis, adding to the complexity for the role of GABAergic signaling in the regulation of these systems. Here we review the effects that stress has on GnRH neurons and HPG axis function alongside evidence supporting GABAARs as a major interface between the stress and reproductive axes.

Keywords: Neurosteroids, stress, GABAA receptors, KCC2, GnRH, estrous, puberty, LH, allopregnanolone, THDOC, tonic inhibition, extrasynaptic, Gabrd

I. INTRODUCTION

The high energy cost of mammalian reproduction and offspring care makes the timing of this important event sensitive to a variety of stressors. Indeed, in his initial report on the general adaptation syndrome, Hans Selye detailed significant reproductive deficits in rats that experienced a wide range of stressors, from cold exposure to excessive exercise. It is here that Selye hypothesizes that these effects-which included atrophy of the gonads and cessation of milk production in lactating rat dams, may involve some general stress-associated suppression of reproduction and suggests that activity of the stress axis must compete with and thus inhibit activity of the reproductive axis [1]. Since these observations, the specific mechanisms underlying the pathophysiological effects of stress on reproduction remain largely unknown. Given the significance of the gonadotropin releasing hormone (GnRH) system as the common central regulator of the hypothalamic-pituitary-gonadal (HPG) axis, it is hypothesized that the central effects of varied stressors converge on these neurons to disrupt their activity.

Stress, the body's response to the metabolic, immunological and psychological factors that threaten homeostasis, involves a coordinated cascade of events across the hypothalamic-pituitary-adrenal (HPA) axis. During the acute stress response, secretion of corticotropin releasing hormone (CRH) from parvocellular neurons of the hypothalamus evokes increased synthesis and release of adrenocorticotropin releasing hormone (ACTH) from the pituitary that acts in an endocrine fashion to induce release of mineralocorticoids and glucocorticoids from the adrenal cortex. These steroid hormones have a host of downstream effects that include cessation of this stress signal. Activation of this HPA axis also involves an increase in the synthesis and release of neuroactive derivative of steroid hormones, termed neurosteroids. This peripheral increase in neurosteroids contributes to the central increase of the 5α reduced neurosteroids tetrahydrodeoxycorticosterone (THDOC) and allopregnanolone (ALLO). THDOC and ALLO are potent modulators of GABAARs, acting preferentially at extrasynaptic, δ subunit-containing GABAARs that mediate tonic inhibition [2, 3]. These receptors play a critical role in the regulation of the HPA axis [4] and have been implicated in the regulation of the HPG axis [5, 6]. Here we review the role for GABAARs, in particular neurosteroid modulation of δ subunit-containing GABAARs, in the regulation of the HPA and HPG axes. Further, we review the evidence that stress disrupts HPG function highlighting a potential role for neurosteroids and their interaction with GABAA receptors. Given significant differences in the physiological characteristics of the HPG axis across species and sexes, this review focuses on findings from female rodents to build a cohesive model supporting GABAA receptors as an interface of the stress and reproductive axes.

II. GABAergic regulation of the HPA axis

The physiological response to stress is mediated by the HPA axis, the activity of which is governed by CRH neurons in the paraventricular nucleus (PVN) of the hypothalamus. CRH neurons are at the apex of HPA axis control, initiating the release of CRH that signals the release of ACTH from the pituitary, which triggers the secretion of cortisol from the adrenal gland in humans (corticosterone in mice). CRH neurons integrate information from many different brain regions involving numerous neurotransmitter systems, but the activity of CRH neurons is ultimately regulated by GABAergic inhibition [for review see 7, 8].

The importance for GABAergic signaling in the regulation of the HPA axis is evident from the fact that nearly 50% of all synapses in the PVN are GABAergic [7, 9]. GABAergic inputs into the PVN arise from local interneurons surrounding the PVN (peri-PVN) as well as from the subparaventricular zone, the anterior hypothalamic area, dorsomedial hypothalamic nucleus, the medial preoptic area, lateral hypothalamic area, and from multiple nuclei within the bed nucleus of the stria terminalis [BNST; 8, 10-12]. In addition, other brain regions exert control over the HPA axis through interneuron mediators [for review see 12].

CRH neurons are regulated by both phasic and tonic GABAergic inhibition, mediated by synaptic and extrasynaptic GABAARs, respectively [13]. Recently, CRH neurons have been shown to be regulated by δ subunit-containing GABAARs [4], which confer sensitivity to neurosteroid modulation [2, 14-17]. When exogenously administered to non-stressed mice the stress-derived neurosteroid, THDOC (10mM), decreases the activity of CRH neurons via actions on δ subunit-containing GABAARs [4]. These results were somewhat unexpected given that autoradiography and immunohistochemistry techniques failed to support a high expression of δ protein in the hypothalamus, especially when compared to regions like the thalamus and cerebellum, where δ is enriched [18-22]. However, combined electrophysiological and pharmacological techniques support tonic GABAergic inhibition in this area mediated by δ containing GABAA receptors [4]. Indeed, reduced responsivity of CRH neurons to δ-targeting compounds has been demonstrated in the hypothalamus of global and CRH specific δ knockout mice [4, 22, 23]. Further, immunoreactivity protocols focused on signal enhancement reveals positive δ expression in the PVN [4, 23]. However, it is important to note that it remains somewhat controversial whether the tonic GABAergic current in parvocellular neurons in the PVN is mediated by GABAAR δ-subunit containing receptors.

Interestingly, the GABAergic regulation of CRH neurons is fundamentally altered following acute stress. Acute restraint stress results in excitatory actions of GABA on CRH neurons due to a collapse in the chloride gradient and a shift in the reversal potential for GABAA (EGABA) [4, 28]. Following acute stress, there is dephosphorylation of KCC2 residue Ser940, which regulates surface expression and function of this transporter [24] and a downregulation of KCC2 in the PVN. KCC2 maintains low intracellular chloride levels in the adult brain, which is essential for the inhibitory actions of GABA [25-27]. Thus, following acute stress, the downregulation of KCC2 results in excitatory actions of GABA on CRH neurons and compromised GABAergic constraint on these neurons [4, 28]. Due to excitatory actions of GABA on CRH neurons, THDOC increases the activity of CRH neurons and increases the corticosterone levels following stress [4]. These excitatory effects of GABA following acute stress are specific for CRH neurons in the PVN since similar changes are not observed in other brain regions, such as the hippocampus [4]. Interestingly, following chronic stress, deficits in GABAergic inhibition are observed in extrahypothalamic regions and have been proposed to contribute to increased hippocampal excitability [29]. These findings demonstrate that the GABAergic regulation and neurosteroid modulation of the HPA axis are state-dependent. This topic has been reviewed in more detail elsewhere [8, 30, 31].

III. GABAergic regulation of the HPG axis

The HPG axis is controlled by GnRH neurons, which consist of just over 1000 neurons scattered across the hypothalamus. About 70% of the projections of these neurons terminate in the median eminence where GnRH can be released into the hypohyseal portal blood system and act as the central regulator of the HPG axis [32, 33]. The pattern of activity of these GnRH neurons is integral for the increase in release of the gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH), from the pituitary gland [34]. Thus, activity of GnRH neurons is coupled to release, which is tightly regulated by GABAergic signaling (Han, 2004; Moenter, 2005). Analyses of inhibitory inputs along GnRH neurons using vGAT expression reveal greater vGAT positive contacts along the GnRH initial dendrite than the soma [35], allowing possible multi-dendritic inhibitory signals from a single inhibitory afferent. Indeed, evidence supporting bundled dendrites coming into contact with a shared synaptic input has been demonstrated in anatomical findings from electron microscopy techniques [36]. These shared-synapses along the dendrites could have significant effects on release of the peptide, as dendrites of GnRH neurons terminating in the median eminence are able to initiate and propagate action potentials [37-39]. However, computational modeling suggests that significant effects of inhibitory inputs to GnRH neurons would involve inhibitory contacts nearer the soma rather than at distal dendrites or terminals at the level of the median eminence [40].

GnRH neurons have been shown to be regulated by both tonic and phasic GABAergic inhibition. These neurons exhibit a tonic GABAergic current sensitive to modulation by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP), an agonist with high efficacy and selectivity for δ subunit-containing GABAARs [6, 15, 41-43]. This THIP-sensitive tonic current is indicative of involvement of extrasynaptic δ subunit-containing GABAARs in regulation of GnRH neurons and is further supported by δ subunit mRNA expression in these cells [6]. In addition to their high sensitivity to THIP, δ containing GABAA receptors have high sensitivity to neurosteroids. ALLO (500nm-5uM) has been shown to alter the resting membrane potential of GnRH neurons [44] as well as augment the depolarizing response of GnRH neurons to GABA [45]. However, these concentrations of ALLO would potentiate the effect of GABA at most GABAAR subtypes [46] and is thus not specific for δ subunit-containing GABAARs. Still, a recent report using a GnRH secreting immortalized mouse hypothalamic cell line found that a synthetic δ GABAAR agonist (DS1) activated GnRH neurons [5]. This GABAergic control of GnRH neurons can influence reproductive function in several ways, including regulation of the estrous cycle and the initiation of puberty [47-49]. Despite the importance of GABAergic signaling in the control of GnRH neurons, it is interesting to note that the loss of the GABAAR γ2 subunit has very little effect on reproductive function [50]. However, the importance of extrasynaptic GABAARs in the regulation of reproductive function remains to be determined.

In female mice and rats, an increase in GnRH multiunit activity precedes the LH surge on the morning of proestrus. Distinct vesicle sorting of the gonadotropins underlies a delayed GnRH-mediated surge in plasma FSH [34]. Given the tight coupling of GnRH activity and LH release, plasma levels of the latter are often used as a surrogate for the former in many studies. Puberty, the period associated with maturation of the above-described reproductive system, may be said to also depend on a maturation of the secretory profile of GnRH, as the mature burst activity profile of GnRH neurons comes online at this time to complete this neuroendocrine circuit. For rodents, this maturation is characterized by a gradual increase in the frequency of GnRH neuron pulse, from every 90 minutes during the peripubertal period to every 30 minutes after vaginal opening and complete pubertal maturation [51]. This switch to the pubertal/mature pulse profile of the GnRH system is thought to involve both downregulation of the inhibitory drive to these neurons and an increase in excitatory inputs [47].

It is important to note that there is some controversy regarding the direction of GABAergic signaling in GnRH neurons. Early pharmacology studies supported an inhibitory role for GABA in the regulation of the HPG axis, with a negative relationship between GABA activity and GnRH or LH activity. Indeed, the LH surge that precedes ovulation is accompanied by a significant decrease in GABA concentration surrounding GnRH cell bodies located in the preoptic area [52] and systemic administration of a GABAAR agonist inhibits the LH surge [53]. Still, depolarizing responses to GABA have been demonstrated for GnRH neurons in slices from adult female mice [54, 55] and from neuronal cultures of adult female rats [56]. Heterogeneous GABAergic responses have been demonstrated in vitro [54, 56-60] and in vivo [61]. Moreover, application of either a GABAAR antagonist or channel blocker has been shown to reduce firing rate of GnRH neurons in female mice [59, 61]. Another report found an increase in the firing rate of GnRH neurons after bath application of a GABAA receptor antagonist [58], which could be due to disinhibition of glutamatergic afferents in the slice, as glutamate receptor antagonists were not included in the slice preparation [62]. Excitatory actions of GABA on GnRH neurons are supported by evidence that the reversal potential of GABA is more positive than the resting potential for these GnRH neurons [54, 60]. These studies support excitatory actions of GABA on GnRH neurons, but also highlight the heterogeneity of this effect. Diversity in the responses of GnRH neurons to GABAAR activation may be due to the fact that two-times as many GnRH neurons express NKCC1 mRNA than KCC2 mRNA, which supports high basal intracellular chloride in these neurons [54]. Although these putative NKCC1 transporters were shown to be expressed by fewer than one quarter of GnRH neurons, the NKCC1 blocker (bumetanide) significantly dampened the stimulatory response of GABA in GnRH neurons [56]. These data highlight the complex role of GABA in the regulation of GnRH neurons and the HPG axis under basal conditions. The relationship of GABAergic control of GnRH neurons becomes increasingly more complex when considering the impact of stress on the HPG axis.

IV. Impact of Stress on the HPG axis: role of GABAARs

Following Selye's original observations, additional reports confirmed a modulatory effect of HPA axis activity on the HPG axis. Of course, the direction of this stress effect may depend on the timing of the stressor exposure, the particular reproductive endpoint of interest, stressor type and the state of the hormonal milieu at the initiation of the stress.

Although one-third of GnRH neurons are thought to express CRH receptors [63], the diverse classes of stressors shown to suppress GnRH activity and associated indices of reproductive health and fertility are not all mediated by activity at CRH. For example, although lesions of the medial amygdala can dampen the restraint stress-induced increase in LH pulse interval, it does not affect similar LH deficiency seen after immunological or metabolic stressors [64]. Similarly, though evidence supporting glucocorticoid regulation of GnRH mRNA through inclusion of a negative glucocorticoid responsive element on the GnRH promoter have been demonstrated from in vitro [65, 66] and in vivo [67] models, chronic corticosterone failed to disrupt LH pulse frequency or amplitude in ovariectomized rats [68]. These findings suggest that psychological and physiological stressors may differentially affect HPG function through distinct neural circuits.

A long history of research supports GABA's role in modulating GnRH physiology. Yet, only a handful of recent investigations implicate GABAAR signaling in the effects of stress on this level of the reproductive axis. Thus far, only one study supports a direct relationship between GABAAR activity in the mPOA and stress effects on GnRH activity (via increased LH interpulse interval). Using pharmacological manipulation of GABAA and GABAB receptors in the mPOA, this work showed a differential role of these GABAAR subtypes in the effects of immunological versus restraint stress [69]. The effect of stress on GnRH neurons and associated reproductive events (puberty and estrous cycle) and evidence supporting role for GABAergic signaling in these effects are summarized in Table 1 and reviewed in detail below.

2.1. Stress, GABAARs, and puberty

Stress may be associated with either delayed or advanced pubertal onset for humans. In particular, children who have experienced early peri/postnatal stress show precocious pubertal development [70, 71], whereas those who have experienced juvenile or peripubertal stress, show a delay. This bidirectional effect of stress on pubertal onset may be said to depend on whether the stress occurs during the infantile period of GnRH neuronal activity or during the quiescent period of inactivity that precedes puberty for primates.

Juvenile rodents do not display a quiescent period of GnRH activity [57, 72]. This may be why, for rats and mice, both perinatal and peripubertal stressors and stress hormone exposure are most often associated with delayed pubertal onset. For example, female rats receiving continuous, intracerebroventricular (icv) administration of CRH beginning at P28, show a dose dependent delay in both vaginal opening and first estrous [73]. Similarly, early life stress, including neonatal stress hormone exposure, is negatively correlated with the delayed onset of both variables associated with sexual maturation in female rats [74] and mice [75, 76]. This delay may also be seen following metabolic stress, as forced fasting is a well-known model of pubertal delay in rats [77]. However, some potent stressors, like maternal separation, do fail to induce changes in pubertal onset [78, 79]. Furthermore, some stressors have been associated with precocious pubertal onset in rodents. Offspring of low-licking/grooming dams show both a hyperactive HPA axis and earlier onset of vaginal opening and estrus than the offspring of high-licking/grooming dams [80]. This early pubertal onset has also been reported following overexpression of CRH in the central nucleus of the amygdala [81]. Interestingly, although neonatal exposure to an immunological stressor is associated with a delay in both vaginal opening and first estrous, these effects are only seen when the immunological stress occurs before the second week of gestation [82]. Finally, metabolic stress may result in either pubertal delay [83, 84] or advanced puberty [85], depending upon whether the stress is associated with an energy surfeit or deficit [85, though see 86].

Fewer studies demonstrate the accompanying effects of these stressors on the pubertal maturation of GnRH activity in rodents. It may be inferred that the stressors associated with pubertal delay also defer the increased activity of GnRH neurons that precede sexual maturation and that stressors associated with precocious puberty do the opposite. Indeed, the GnRH interpulse interval in hypothalamic explants of female rats that experienced neonatal undernutrition with delayed pubertal onset is relatively prolonged, much more similar to the interpulse interval seen during the juvenile stage [87]. Furthermore, the early onset of puberty seen following the metabolic stress associated with high fat diet is accompanied by an early increase in LH pulse frequency [88]. Further efforts are required to define the relationship between stress, pubertal delay and GnRH neuronal activity, as well as the role of GABAergic signaling in mediating these effects.

Though activity of GABAARs is thought to play a role in the maturation of the HPG axis [89, 90], very few studies have investigated changes in the GABAergic control of GnRH neurons, or the HPG axis, following stress in the pre-pubertal system. One study on the HPG response to prepubertal (P15) and peripubertal (P30) exposure to an acute dose of bacterial endotoxin (LPS) found a reduction in hypothalamic GABA content and plasma LH only after peripubertal exposure to this metabolic stressor [91]. Chronic exposure to this metabolic stressor was also found to also cause a reduction in GABA content and plasma LH as well as a delay vaginal opening by 4 days [92]. Further research is required to fully appreciate the role of GABAergic signaling in mediating the effects of stress on GnRH neurons in the pubertal transition.

2.2. Stress, GABAARs, and mature HPG function

The mature HPG axis is also vulnerable to stress. However, the effects of stress on the estrous cycle, GnRH/LH activity, and fertility depend strongly on the type of stressor. Therefore, the effects of different types of stressors (metabolic, immunological, or psychogenic) on HPG function are reviewed separately below.

2.2.1 Metabolic stressors

Fasting in mice may disrupt regular estrous cycling by causing a delay in the transition to estrus, when the fasting is initiated during diestrus [93]. This metabolic stressor is associated with a dampened GnRH pulse profile [94]. Although the GnRH neurons of fasted mice still display action potentials in response to locally applied GABA (mM) and a reduction in the frequency of GABAergic sPSCs, these neurons also show an increase in the GABAergic postsynaptic response to leptin (50mM), including an increase in the rise time, amplitude and decay of sPSCs [94]. As current evidence suggests that leptin does not directly act on GnRH neurons through classic cascades associated with leptin receptor activation [95], it is possible that the enhanced GABAergic postsynaptic response to leptin following fasting still involves some change in the GABAAR population on these GnRH neurons. Indeed, in the immature system, leptin has been shown to potentiate GABAergic-PSCs through an increase in the expression of functional GABAergic synapses as well as an induction of multiple signaling molecules thought to be involved in synaptic plasticity [96]. Thus, the effects of fasting-stress on GnRH activity may involve both changes in presynaptic inhibitory drive and alterations in the GABAAR profile on GnRH neurons. As such, mice with disrupted leptin signaling to GABAergic neurons show delayed puberty onset, reduced plasma LH and proestrus incidence.

Unlike fasting, metabolic stress associated with high fat intake results in an increase in GnRH secretion in mice [97]. This increased hypothalamic activity also augments plasma LH and disrupts regular cycling. Though female rats on a high fat diet also display an increase in irregular cycling, this is associated with a reduction in LH activity, much like fasting [98]. These findings highlight potential differences in the regulatory signals of GnRH activity across species. There is a gap in the literature regarding a role for GABAARs in the effects of high fat diet on GnRH activity. However, GABAergic signaling has been shown to be depressed in the dorsomedial hypothalamus of male rats on a high fat diet [99]. This hypothalamic nucleus is believed to be the location of the GnRH neurons integral to GnRH pulse generation, as their activity is most strongly associated with an exogenously induced LH surge [100] and deafferentation of this region from the rest of the brain disrupts ovulation [101]. High fat diet also reduces total GABAA binding densities in the hypothalamus of male rats [102]. Further efforts are needed to determine whether the effects of high fat diet on the GABAergic system may be extended to females and whether they directly impact GnRH neurons.

2.2.2 Immunological stressors

The effects of some stressors (such as intermittent cold-stress) on GnRH activity are accompanied by changes in cytokine expression [103], which may have an impact on reproductive events. Indeed, administration of a bacterial endotoxin (LPS) that potently induces an inflammatory stress response, including activation of proinflammatory cytokines, consistently suppresses LH pulse frequency [104-106]. This effect may be recapitulated following administration of cytokines themselves [106-108]. Furthermore, the reduced LH pulse frequency may be accompanied by a tempering of the increased GnRH multiunit activity that usually precedes the LH surge [106]. There is also a reduction in the surge in GnRH transcription that occurs during the proestrus phase in rats [109]. Although immunological stressors have potent and fast inhibitory effects on GnRH activity, and the effects of chronic immunological stress on estrous cycling has been established in rats [110], lasting changes in the behavioral events associated with acute reduction in GnRH activity may depend on the state of the hormonal milieu at the time of the stressor. For example, an increase in estrous cycle irregularity following LPS administration was found to most consistently occur when LPS was administered during diestrus [111].

Activation of GABAARs on GnRH neurons by elements associated with these immunological stressors may underlie the fast inhibitory effect of these stressors on HPG activity. Indeed, infusion of a GABAA receptor antagonist (bicuculline) into the mPOA blocks the effect of LPS on LH pulse frequency [105]. Furthermore, Il-1β, a pro-inflammatory cytokine, has been shown to augment α5-mediated tonic current in the hippocampus [112]. As mRNA for this subunit was shown to be the most consistently expressed GABAAR transcript in single GnRH positive neurons harvested for analysis from GFP-GnRH mice [113], similar effects may occur in GnRH neurons. Thus, the sensitivity of tonic inhibitory current to this integral inflammatory cytokine suggests that immunological stressors may modulate GnRH activity by potentiating the tonic current mediated by extrasynaptic GABAARs exerting a hyperpolarizing effect on the resting membrane potential of GnRH neurons, suppressing the release of the peptide.

2.2.3 Psychogenic stressors

Restraint stress has been shown to block the surge in LH associated with the evening of proestrus in mice [114], block or dampen both the proestrus LH surge and ovulation in cycling rats [115] and prolong the time between LH pulses in estrogen-primed ovariectomized rats [105]. This delay in LH pulse is maintained even when the restraint is preceded by an intra-mPOA infusion of the GABAAR antagonist bicuculline [105]. Although this suggests that GABAARs may not play a role in mediating the effects of this stressor on GnRH activity, it is possible that the bicuculline treatment is acting to alter the activity of afferents to GnRH neurons in this area in addition to blocking GABAARs on the GnRH neurons themselves. Indeed, like fasting stress, restraint stress may be associated with changes in presynaptic GABAergic drive to GnRH neurons, as this treatment is positively associated with activation of GAD67 cells in regions populated by GnRH neurons [116]. Though an increase in GAD transcription has been shown in the POA of chronically stressed male rats [117], this has not been confirmed for females. GAD immunoreactivity is enhanced for female rats that have received infusions of CRH into the BNST [118]. This procedure is meant to mimic the CRH activation that may accompany stressors like restraint, and has been shown to reduce the frequency (and increase the interpulse interval) of LH pulses [118]. Preliminary reports suggest that, in addition to presynaptic changes in GABAergic signaling to GnRH neurons, central increases in CRH may have postsynaptic effects on GABAARs. For example, CRH (50nM) was shown to decrease the frequency of GABAergic sIPSCs [119; conference abstract]. More efforts are needed to determine whether the effects of psychogenic stress in these rodent models affect GABAAR signaling on GnRH neurons.

Restraint stress may also regulate activity of GABAARs on GnRH neurons by shifting the expression of NKCC1 and KCC2. As mentioned earlier, downregulation of KCC2 in CRH releasing parvocellular neurons of the hypothalamus play an integral role in the neuroendocrine and behavioral response to acute stress [4]. Similarly, downregulation of KCC2 in the PVN occurs following chronic social defeat stress [120]. This downregulation also occurs in the hippocampus following chronic restraint stress [29]. Therefore, reproductive effects associated with restraint stress may involve changes in KCC2 expression on GnRH neurons and a change in the postsynaptic response of GABAARs on these cells. The effects of stress on chloride homeostasis and the direction of GABAergic signaling and impact on HPG function remain to be determined.

There is also very little known about the contribution of specific GABAAR subtypes in mediating the effects of stress on mature HPG function. Interestingly, loss of the GABAAR γ2 subunit, which mediates the phasic component of GABAergic signaling, has very little effect on reproductive function [50]. However, the role of these receptors in mediating the effects of stress on HPG function has not been explored. Similarly, to-date, no studies have investigated the role for extrasynaptic GABAARs, such as those containing the δ subunit, in mediating the effects of stress on the HPG axis. Nor has there been an investigation into the role of neurosteroid actions on GABAARs, in particular stress-derived neurosteroids, in the effects of stress on reproductive function.

V. Concluding Remarks

Stress negatively impacts reproductive function. However, much remains to be learned regarding the mechanisms whereby stress alters HPG function. Stress alters the activity of GnRH neurons and disrupts reproductive events, including the pubertal transition and the estrous cycle. GABAARs have been shown to regulate GnRH activity with recent reports supporting a role for δ subunit-containing GABAAR-mediated tonic inhibition on these neurons. GABAARs incorporating the δ subunit display higher sensitivity to neurosteroids, including stress-derived neurosteroids. Thus, δ subunit-containing GABAARs may be perfectly poised to regulate crosstalk between the stress and reproductive axes. Future studies will explore the role of extrasynaptic GABAARs, tonic inhibition, and neurosteroid modulation in mediating the effects of stress on HPG function.

HIGHLIGHTS.

  • Synaptic and extrasynaptic GABAARs play a role in the regulation of corticotropin releasing hormone (CRH) neurons and the stress axis

  • Neurosteroid actions on GABAARs may modulate CRH neuron activity and stress axis function

  • Gonadotropin releasing hormone (GnRH) neurons are regulated by a THIP-sensitive tonic GABAergic current, which likely contributes to the regulation of the reproductive axis

  • Stress-derived neurosteroids via actions on GABAARs may mediate the effects of stress on the reproductive axis

Acknowledgments

J.M. is supported by NIH-NINDS grant R01 NS073574 (J.M.). L.C.M is supported by NIH-NIGMS grant K12GM074869; an IRACDA training grant to Tufts University, Training in Education and Critical Research Skills (TEACRS).

Footnotes

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