Direct regulation of postnatal GnRH neurons by the progesterone derivative allopregnanolone in the mouse

Mating Enhances Expression of Hormonal and Trophic Factors in the Midbrain of Female Rats

Among female rats, mating enhances neurosteroid formation in the midbrain ventral tegmental area (VTA; independent of peripheral steroid-secreting glands, ovaries, and adrenals). The sources/targets for these actions are not well understood. In Experiment 1, proestrous rats engaged in a mating paradigm, or did not, and the midbrains had been assessed via the Affymetrix rat genome microarrays. In Experiment 2, the influence of gonadal and adrenal glands on the expression of these genes was assessed in rats that were proestrous, ovariectomized (OVX), or OVX and adrenalectomized (ADX). The microarrays revealed 53 target genes that were significantly up-regulated (>2.0-fold change) in response to mating. Mating significantly enhanced the midbrain mRNA expression of genes involved in hormonal and trophic actions: Gh1, S100g, and Klk1b3 in proestrous, but not OVX and/or ADX, rats; Fshb in all but OVX/ADX rats; and lutenizing hormone β and thyroid-stimulating hormone (TSH) β in all rats. Thus, mating enhances midbrain gene expression independent and dependent of peripheral glands.

Neurosteroid Actions in Memory and Neurologic/Neuropsychiatric Disorders

Memory dysfunction is a symptomatic feature of many neurologic and neuropsychiatric disorders; however, the basic underlying mechanisms of memory and altered states of circuitry function associated with disorders of memory remain a vast unexplored territory. The initial discovery of endogenous neurosteroids triggered a quest to elucidate their role as neuromodulators in normal and diseased brain function. In this review, based on the perspective of our own research, the advances leading to the discovery of positive and negative neurosteroid allosteric modulators of GABA type-A (GABA_A), NMDA, and non-NMDA type glutamate receptors are brought together in a historical and conceptual framework. We extend the analysis toward a state-of-the art view of how neurosteroid modulation of neural circuitry function may affect memory and memory deficits. By aggregating the results from multiple laboratories using both animal models for disease and human clinical research on neuropsychiatric and age-related neurodegenerative disorders, elements of a circuitry level view begins to emerge. Lastly, the effects of both endogenously active and exogenously administered neurosteroids on neural networks across the life span of women and men point to a possible underlying pharmacological connectome by which these neuromodulators might act to modulate memory across diverse altered states of mind.

The neuroendocrine genesis of polycystic ovary syndrome: A role for arcuate nucleus GABA neurons

Polycystic ovary syndrome (PCOS) is a prevalent and distressing endocrine disorder lacking a clearly identified aetiology. Despite its name, PCOS may result from impaired neuronal circuits in the brain that regulate steroid hormone feedback to the hypothalamo-pituitary-gonadal axis. Ovarian function in all mammals is controlled by the gonadotropin-releasing hormone (GnRH) neurons, a small group of neurons that reside in the pre-optic area of the hypothalamus. GnRH neurons drive the secretion of the gonadotropins from the pituitary gland that subsequently control ovarian function, including the production of gonadal steroid hormones. These hormones, in turn, provide important feedback signals to GnRH neurons via a hormone sensitive neuronal network in the brain. In many women with PCOS this feedback pathway is impaired, resulting in the downstream consequences of the syndrome. This review will explore what is currently known from clinical and animal studies about the identity, relative contribution and significance of the individual neuronal components within the GnRH neuronal network that contribute to the pathophysiology of PCOS. We review evidence for the specific neuronal pathways hypothesised to mediate progesterone negative feedback to GnRH neurons, and discuss the potential mechanisms by which androgens may evoke disruptions in these circuits at different developmental time points. Finally, this review discusses data providing compelling support for disordered progesterone-sensitive GABAergic input to GnRH neurons, originating specifically within the arcuate nucleus in prenatal androgen induced forms of PCOS.

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

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.

Targeting the modulation of neural circuitry for the treatment of anxiety disorders

Anxiety disorders are a major public health concern. Here, we examine the familiar area of anxiolysis in the context of a systems-level understanding that will hopefully lead to revealing an underlying pharmacological connectome. The introduction of benzodiazepines nearly half a century ago markedly improved the treatment of anxiety disorders. These agents reduce anxiety rapidly by allosterically enhancing the postsynaptic actions of GABA at inhibitory type A GABA receptors but side effects limit their use in chronic anxiety disorders. Selective serotonin reuptake inhibitors and serotonin/norepinephrine reuptake inhibitors have emerged as an effective first-line alternative treatment of such anxiety disorders. However, many individuals are not responsive and side effects can be limiting. Research into a relatively new class of agents known as neurosteroids has revealed novel modulatory sites and mechanisms of action that are providing insights into the pathophysiology of certain anxiety disorders, potentially bridging the gap between the GABAergic and serotonergic circuits underlying anxiety. However, translating the pharmacological activity of compounds targeted to specific receptor subtypes in rodent models of anxiety to effective therapeutics in human anxiety has not been entirely successful. Since modulating any one of several broad classes of receptor targets can produce anxiolysis, we posit that a systems-level discovery platform combined with an individualized medicine approach based on noninvasive brain imaging would substantially advance the development of more effective therapeutics.

Glucocorticoid Regulation of Reproduction

It is well accepted that stress, measured by increased glucocorticoid secretion, leads to profound reproductive dysfunction. In times of stress, glucocorticoids activate many parts of the fight or flight response, mobilizing energy and enhancing survival, while inhibiting metabolic processes that are not necessary for survival in the moment. This includes reproduction, an energetically costly procedure that is very finely regulated. In the short term, this is meant to be beneficial, so that the organism does not waste precious energy needed for survival. However, long-term inhibition can lead to persistent reproductive dysfunction, even if no longer stressed. This response is mediated by the increased levels of circulating glucocorticoids, which orchestrate complex inhibition of the entire reproductive axis. Stress and glucocorticoids exhibits both central and peripheral inhibition of the reproductive hormonal axis. While this has long been recognized as an issue, understanding the complex signaling mechanism behind this inhibition remains somewhat of a mystery. What makes this especially difficult is attempting to differentiate the many parts of both of these hormonal axes, and new neuropeptide discoveries in the last decade in the reproductive field have added even more complexity to an already complicated system. Glucocorticoids (GCs) and other hormones within the hypothalamic-pituitary-adrenal (HPA) axis (as well as contributors in the sympathetic system) can modulate the hypothalamic-pituitary-gonadal (HPG) axis at all levels-GCs can inhibit release of GnRH from the hypothalamus, inhibit gonadotropin synthesis and release in the pituitary, and inhibit testosterone synthesis and release from the gonads, while also influencing gametogenesis and sexual behavior. This chapter is not an exhaustive review of all the known literature, however is aimed at giving a brief look at both the central and peripheral effects of glucocorticoids on the reproductive function.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

Non-classical effects of estradiol on cAMP responsive element binding protein phosphorylation in gonadotropin-releasing hormone neurons: mechanisms and role

Gonadotropin-releasing hormone (GnRH) is produced by a heterogenous neuronal population in the hypothalamus to control pituitary gonadotropin production and reproductive function in all mammalian species. Estradiol is a critical component for the communication between the gonads and the central nervous system. Resolving the mechanisms by which estradiol modulates GnRH neurons is critical for the understanding of how fertility is regulated. Extensive studies during the past decades have provided compelling evidence that estradiol has the potential to alter the intracellular signal transduction mechanisms. The common target of many signaling pathways is the phosphorylation of a key transcription factor, the cAMP response element binding protein (CREB). This review first addresses the aspects of estradiol action on CREB phosphorylation (pCREB) in GnRH neurons. Secondly, this review considers the receptors and signaling network that regulates estradiol's action on pCREB within GnRH neurons and finally it summarizes the physiological significance of CREB to estrogen feedback.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

The role of kisspeptin and RFamide-related peptide-3 neurones in the circadian-timed preovulatory luteinising hormone surge

Many aspects of female reproduction often require intricate timing, ranging from the temporal regulation of reproductive hormone secretion to the precise timing of sexual behaviour. In particular, in rodents and other species, ovulation is triggered by a surge in pituitary luteinising hormone (LH) secretion that is governed by a complex interaction between circadian signals arising in the hypothalamus and ovarian-derived oestradiol signals acting on multiple brain circuitries. These circadian and hormonal pathways converge to stimulate a precisely-timed surge in gonadotropin-releasing hormone (GnRH) release (i.e. positive-feedback), thereby triggering the preovulatory LH surge. Reflecting its control by afferent circadian signals, the preovulatory LH surge occurs at a specific time of day, typically late afternoon in nocturnal rodents. Although the specific mechanisms mediating the hormonal and circadian regulation of GnRH/LH release have remained poorly understood, recent findings now suggest that oestradiol and circadian signals govern specific reproductive neuropeptide circuits in the hypothalamus, including the newly-identified kisspeptin and RFamide-related peptide (RFRP)-3 neuronal populations. Neurones producing kisspeptin, the protein product of the Kiss1 gene, and RFRP-3 have been shown to provide excitatory and inhibitory input to GnRH neurones, respectively, and are also influenced by sex steroid and circadian signals. In the present review, we integrate classic and recent findings to form a new working model for the neuroendocrine regulation of the circadian-timed preovulatory LH surge in rodents. This model proposes kisspeptin and RFRP-3 neuronal populations as key nodal points for integrating and transducing circadian and hormonal signals to the reproductive axis, thereby governing the precisely-timed LH surge.

Allopregnanolone, a GABAA receptor agonist, decreases gonadotropin levels in women. A preliminary study

Animal studies suggest regulatory effects on the hypothalamic-pituitary-gonad axis by allopregnanolone, an endogenous gamma-aminobutyric acid A (GABA(A)) receptor agonist. Elevated levels of allopregnanolone in women with hypothalamic amenorrhea have been seen. Isoallopregnanolone is an isomer to allopregnanolone, but without GABA(A) receptor effects. The purpose of this study was to investigate effects of allopregnanolone and isoallopregnanolone on gonadotropin levels in healthy women of fertile age. Ten women were given allopregnanolone and five women isoallopregnanolone intravenously in follicular phase. Repeated blood samples were drawn during the test day. Main outcomes were changes in serum levels of follicle-stimulating hormone (FSH), luteinising hormone (LH), oestradiol, and progesterone. Serum-FSH decreased between 5 and 105 min after the allopregnanolone injection (F(16,144)=2.18, p=0.008). Serum-LH was reduced between 5 and 35 min following the allopregnanolone injection (F(16,144)=2.63, p=0.001). Serum-oestradiol and -progesterone were not significantly changed after allopregnanolone injections. No effect on gonadotropin levels were seen after administration of isoallopregnanolone. Allopregnanolone reduces FSH and LH levels in women and the effect might be mediated via a specific GABA(A) receptor activation since isoallopregnanolone lacked this effect. Although the number of women was small, the results suggest a regulatory mechanism on the hypothalamic-pituitary-gonadal axis by allopregnanolon.

Novel substrates for, and sources of, progestogens for reproduction

Steroid hormones, such as progesterone, are typically considered to be primarily secreted by the gonads (albeit adrenals can also be a source) and to exert their actions through cognate intracellular progestin receptors (PRs). Through its actions in the midbrain ventral tegmental Area (VTA), progesterone mediates appetitive (exploratory, anxiety, social approach) and consummatory (social, sexual) aspects of rodents' mating behaviour. However, progesterone and its natural metabolites ('progestogens') are produced in the midbrain VTA independent of peripheral sources and midbrain VTA of adult rodents is devoid of intracellular PRs. One approach that we have used to understand the effects of progesterone and mechanisms in the VTA for mating is to manipulate the actions of progesterone in the VTA and to examine effects on lordosis (the posture female rodents assume for mating to occur). This review focuses on the effects and mechanisms of progestogens to influence reproduction and related processes. The actions of progesterone and its 5α-reduced metabolite and neurosteroid, 5α-pregnan-3α-ol-20-one (3α,5α-THP; allopregnanolone) in the midbrain VTA to facilitate mating are described. The findings that 3α,5α-THP biosynthesis in the midbrain occurs with mating are discussed. Evidence for the actions of 3α,5α-THP in the midbrain VTA via nontraditional steroid targets is summarised. The broader relevance of these actions of 3α,5α-THP for aspects of reproduction, beyond lordosis, is summarised. Finally, the potential role of the pregnane xenobiotic receptor in mediating 3α,5α-THP biosynthesis in the midbrain is introduced.

Allopregnanolone induces LHRH and glutamate release through NMDA receptor modulation

LHRH release from hypothalamus is influenced by the neurotransmitter glutamate that acts, among others, on NMDA receptors present in LHRH neurons. On the other hand, the neurosteroid allopregnanolone can modulate the activity of specific neurotransmitter receptors and affect neurotransmitter release. We examined the role of allopregnanolone on in vitro LHRH and glutamate release from mediobasal hypothalamus and anterior preoptic area of ovariectomized rats with estrogen and progesterone replacement. Moreover, we evaluated whether the neurosteroid might act through modulation of NMDA receptors. Allopregnanolone induced an increase in LHRH release. This effect was reversed when the NMDA receptors were blocked by the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (AP-7) indicating that this neurosteroid would interact with NMDA receptors. Moreover allopregnanolone induced an augment in K(+) evoked [(3)H]-glutamate release from mediobasal hypothalamus-anterior preoptic area explants and this effect was also reversed when NMDA receptors were blocked with AP-7. These results suggest an important physiologic function of allopregnanolone on the regulation of neuroendocrine function in female adult rats. Not only appears to be involved in enhancing LHRH release through modulation of NMDA receptors but also in the release of glutamate which is critical in the control of LHRH release.

Depolarising and hyperpolarising actions of GABA(A) receptor activation on gonadotrophin-releasing hormone neurones: towards an emerging consensus

The gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones of a complex neuronal network that controls fertility. It is now appreciated that GABAergic neurones within this network provide an important regulatory influence on GnRH neurones. However, the consequences of direct GABA(A) receptor activation on adult GnRH neurones have been controversial for nearly a decade now, with both hyperpolarising and depolarising effects being reported. This review provides: (i) an overview of GABA(A) receptor function and its investigation using electrophysiological approaches and (ii) re-examines the past and present results relating to GABAergic regulation of the GnRH neurone, with a focus on mouse brain slice data. Although it remains difficult to reconcile the results of the early studies, there is a growing consensus that GABA can act through the GABA(A) receptor to exert both depolarising and hyperpolarising effects on GnRH neurones. The most recent studies examining the effects of endogenous GABA release on GnRH neurones indicate that the predominant action is that of excitation. However, we are still far from a complete understanding of the effects of GABA(A) receptor activation upon GnRH neurones. We argue that this will require not only a better understanding of chloride ion homeostasis in individual GnRH neurones, and within subcellular compartments of the GnRH neurone, but also a more integrative view of how multiple neurotransmitters, neuromodulators and intrinsic conductances act together to regulate the activity of these important cells.

Tonic extrasynaptic GABA(A) receptor currents control gonadotropin-releasing hormone neuron excitability in the mouse

It is well established that the GABA(A) receptor plays an important role in regulating the electrical excitability of GnRH neurons. Two different modes of GABA(A) receptor signaling exist: one mediated by synaptic receptors generating fast (phasic) postsynaptic currents and the other mediated by extrasynaptic receptors generating a persistent (tonic) current. Using GABA(A) receptor antagonists picrotoxin, bicuculline methiodide, and gabazine, which differentiate between phasic and tonic signaling, we found that ∼50% of GnRH neurons exhibit an approximately 15-pA tonic GABA(A) receptor current in the acute brain slice preparation. The blockade of either neuronal (NO711) or glial (SNAP-5114) GABA transporter activity within the brain slice revealed the presence of tonic GABA signaling in ∼90% of GnRH neurons. The GABA(A) receptor δ subunit is only found in extrasynaptic GABA(A) receptors. Using single-cell RT-PCR, GABA(A) receptor δ subunit mRNA was identified in GnRH neurons and the δ subunit-specific agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol was found to activate inward currents in GnRH neurons. Perforated-patch clamp studies showed that 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol exerted the same depolarizing or hyperpolarizing effects as GABA on juvenile and adult GnRH neurons and that tonic GABA(A) receptor signaling regulates resting membrane potential. Together, these studies reveal the presence of a tonic GABA(A) receptor current in GnRH neurons that controls their excitability. The level of tonic current is dependent, in part, on neuronal and glial GABA transporter activity and mediated by extrasynaptic δ subunit-containing GABA(A) receptors.

The neurosteroid allopregnanolone modulates specific functions in central and peripheral glial cells

Since the first observations on the existence of "neurosteroids" in the 1980s, our understanding of the importance of these endogenous steroids in the control of the central and peripheral nervous system (PNS) has increased progressively. Although most of the observations were made in neuronal cells, equally important are the effects that neurosteroids exert on glial cells. Among the different classes of neurosteroids acting on glial cells, the progesterone 5α-3α metabolite, allopregnanolone, displays a particular mechanism of action involving primarily the modulation of classic GABA receptors. In this review, we focus our attention on allopregnanolone because its effects on the physiology of glial cells of the central and PNS are intriguing and could potentially lead to the development of new strategies for neuroprotection and/or regeneration of injured nervous tissues.

Regulation of GNRH production by estrogen and bone morphogenetic proteins in GT1-7 hypothalamic cells

Recent studies have shown that bone morphogenetic proteins (BMPs) are important regulators in the pituitary-gonadal endocrine axis. We here investigated the effects of BMPs on GNRH production controlled by estrogen using murine GT1-7 hypothalamic neuron cells. GT1-7 cells expressed estrogen receptor alpha (ERalpha; ESR1 as listed in MGI Database), ERbeta (ESR2 as listed in MGI Database), BMP receptors, SMADs, and a binding protein follistatin. Treatment with BMP2 and BMP4 had no effect on Gnrh mRNA expression; however, BMP6 and BMP7 significantly increased Gnrh mRNA expression as well as GnRH production by GT1-7 cells. Notably, the reduction of Gnrh expression caused by estradiol (E(2)) was restored by cotreatment with BMP2 and BMP4, whereas it was not affected by BMP6 or BMP7. E(2) activated extracellular signal-regulated kinase (ERK) 1/2 and stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK) signaling but did not activate p38-mitogen-activated protein kinase (MAPK) signaling in GT1-7 cells. Inhibition of ERK1/ERK2 reversed the inhibitory effect of estrogen on Gnrh expression, whereas SAPK/JNK inhibition did not affect the E(2) actions. Expression levels of Eralpha and Erbeta were reduced by BMP2 and BMP4, but were increased by BMP6 and BMP7. Treatment with an ER antagonist inhibited the E(2) effects on Gnrh suppression including reduction of E(2)-induced ERK phosphorylation, suggesting the involvement of genomic ER actions in Gnrh suppression. BMP2 and BMP4 also suppressed estrogen-induced phosphorylation of ERK1/ERK2 and SAPK/JNK signaling, suggesting that BMP2 and BMP4 downregulate estrogen effects by attenuating ER-MAPK signaling. Considering that BMP6 and BMP7 increased the expression of alpha1E-subunit of R-type calcium channel (Cacna1e), which is critical for GNRH secretion, it is possible that BMP6 and BMP7 directly stimulate GNRH release by GT1-7 cells. Collectively, a newly uncovered interaction of BMPs and ER may be involved in controlling hypothalamic GNRH production and secretion via an autocrine/paracrine mechanism.

Steroid Hormones and Growth Factors Act in an Integrated Manner at the Levels of Hypothalamic Astrocytes

Several growth factors (e.g., transforming growth factors beta and alpha, basic fibroblast growth factor), produced by hypothalamic astrocytes, participate in the control of hypothalamic gonadotrophin-releasing hormone (GnRH) neurons. On this basis, we have hypothesized that steroid hormones, like estrogens and progestagens, influence the GnRH neurons by modulating in glial cells the synthesis and the release of these growth factors. Data reported here indicate that the expression of transforming growth factor beta 1 is modulated in hypothalamic astrocytes by a progesterone derivative (i.e., dihydroprogesterone), while estrogens modulate that of basic fibroblast growth factor. Moreover, it is interesting to highlight that the effect of estrogens on basic fibroblast growth factor is mediated by another growth factor (i.e., transforming growth factor alpha). Altogether, the present findings support the concept that steroid hormones and growth factors act in an integrated manner at the level of hypothalamic astrocytes, thus adding a further piece of knowledge in the understanding of the mechanisms controlling GnRH neurons.

Neurosteroids alter gamma-aminobutyric acid postsynaptic currents in gonadotropin-releasing hormone neurons: a possible mechanism for direct steroidal control

Pulsatile GnRH release is required for fertility and is regulated by steroid feedback. Whether or not steroids or their metabolites act directly on GnRH neurons is not well established. In some neurons, steroid metabolites known as neurosteroids modulate the function of the GABAA receptor. Specifically, the progesterone derivative allopregnanolone is an allosteric agonist at this receptor, whereas the androgen dehydroepiandrosterone sulfate (DHEAS) is an allosteric antagonist. We hypothesized these metabolites act similarly on GnRH neurons to modify the response to GABA. Whole-cell voltage-clamp recordings of GABAergic miniature postsynaptic currents (mPSCs) were made from green fluorescent protein-identified GnRH neurons in brain slices from diestrous mice. Glutamatergic currents were blocked with antagonists and action potentials blocked with tetrodotoxin, minimizing presynaptic effects of treatments. Allopregnanolone (5 microm) increased mPSC rate of rise, amplitude and decay time by 15.9 +/- 6.1%, 16.5 +/- 6.3%, and 58.3 +/- 18.6%, respectively (n = 7 cells). DHEAS (5 microm) reduced mPSC rate of rise (32.1 +/- 5.7%) and amplitude (27.6 +/- 4.3%) but did not alter decay time (n = 8). Effects of both neurosteroids were dose dependent between 0.1 and 10 microm. In addition to independent actions, DHEAS also reversed effects of allopregnanolone on rate of rise and amplitude so that these parameters were returned to pretreatment baseline values (n = 6). These data indicate allopregnanolone increases and DHEAS decreases responsiveness of GnRH neurons to activation of GABAA receptors by differentially modulating current flow through GABAA receptor chloride channels. This provides one mechanism for direct steroid feedback to GnRH neurons.

Astrocytes and brain function: implications for reproduction

Recent evidence suggests that astrocytes have important neuroregulatory functions in addition to their classic functions of support and segregation of neurons. These newly revealed functions include regulation of neuron communication, neurosecretion, and synaptic plasticity. Although these actions occur throughout the brain, this review will focus on astrocyte-neuron interactions in the hypothalamus, particularly with respect to their potential contribution to the regulation of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Hypothalamic astrocytes have been documented to release a variety of neuroactive factors, including transforming growth factors-alpha and -beta, insulin-like growth factor-1, prostaglandin E2, and the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnane-20-one. Each of these factors has been shown to stimulate GnRH release, and receptors for each factor have been documented on GnRH neurons. Astrocytes have also been implicated in the regulation of synaptic plasticity in key areas of the hypothalamus that control GnRH release, an effect achieved by extension and retraction of glial processes (i.e., glial ensheathment). Through this mechanism, the number of synapses on GnRH neurons and GnRH regulatory neurons can potentially be modulated, thereby influencing the activation state of GnRH neurons. The steroid hormone 17beta-estradiol, which triggers the GnRH and luteinizing hormone surge, has been shown to induce the astrocyte-regulated changes in hypothalamic synaptic plasticity, as well as enhance formation and release of the astrocyte neuroactive factors, thereby providing another potential mechanistic layer for astrocyte regulation of GnRH release. As a whole, these studies provide new insights into the diversity of astrocytes and their potential role in reproductive neuroendocrine function.

Single cell expression analysis--pharmacogenomic potential

A fundamental challenge in biology is to correlate physiology with gene expression in specific cell types. This can only be achieved by understanding gene expression at the level of the single cell because, in many systems, each cell has the capacity to express a unique set of genes. Therefore, each cell can be considered to be functionally distinct. A clearer understanding of gene expression differences at such a discrete level provides an opportunity to develop drugs with more targeted pharmacologies or with decreased side effects.

Effect of GABA on GnRH neurons switches from depolarization to hyperpolarization at puberty in the female mouse

The amino acid gamma-aminobutyric acid (GABA) plays an important role in the regulation of the GnRH neurons. We examined whether GABA depolarizes or hyperpolarizes GnRH neurons over postnatal development using gramicidin, perforated-patch electrophysiology combined with GnRH-LacZ transgenic mice in whom GnRH neurons can be made to fluoresce. The basic membrane properties and GABA responsiveness of GnRH neurons were not altered by transgene expression or fluorescence. Ten of 12 immature GnRH neurons (10-17 d) were depolarized by GABA in a direct and dose-dependent manner that was blocked by a GABA(A) receptor antagonist. In peripubertal GnRH neurons (25-30 d), GABA exerted depolarizing (4/11) as well as hyperpolarizing (5/11) effects on GnRH neurons. In adult female mice, GABA was found to exert exclusively hyperpolarizing actions on GnRH neurons (9/10) that were direct and mediated by the GABA(A) receptor. GABA switched from depolarizing to hyperpolarizing actions around postnatal d 31, the time of vaginal opening. Unidentified preoptic area neurons exhibited predominantly hyperpolarizing responses to GABA at all three postnatal stages. These findings demonstrate that GnRH neurons display an unusually late postnatal switch in their response to GABA. They also provide the first direct evidence that GABA inhibits the electrical activity of postpubertal GnRH neurons.

Molecular and cellular properties of GnRH neurons revealed through transgenics in the mouse

Recent advances in the use of gonadotropin-releasing hormone (GnRH) promoter-driven transgenics in the mouse are beginning to open up the once elusive GnRH neuronal phenotype to detailed molecular and cellular investigation. This review highlights progress in the development of GnRH promoter transgenic constructs and the understanding of murine gene sequences required for the correct temporal and spatial targeting of transgenes to the GnRH phenotype in vivo. Strategies enabling the identification of single, living GnRH neurons in the acute brain slice preparation are allowing gene profiling and electrophysiological experiments to be undertaken. Results so far indicate that, like other neurons, GnRH cells express a variety of sodium, potassium and calcium channels as well as GABAergic and glutamatergic receptors which are responsible for determining the membrane properties and firing characteristics of the GnRH neuron. Many of these receptors and channels appear to be expressed heterogeneously within the GnRH phenotype. Furthermore, several display distinct postnatal developmental expression profiles which are likely to be of consequence to the development of synchronized, pulsatile GnRH secretion in the adult animal.