Seasonal changes in the inputs to gonadotropin-releasing hormone neurones in the ewe brain: an assessment by conventional fluorescence and confocal microscopy

Dopaminergic inhibition of gonadotropin-releasing hormone neurons in the cichlid fish Astatotilapia burtoni

Dopamine regulates reproduction in part by modulating neuronal activity within the hypothalamic-pituitary-gonadal (HPG) axis. Previous studies suggested numerous mechanisms by which dopamine exerts inhibitory control over the HPG axis, ultimately changing the levels of sex steroids that regulate reproductive behaviors. However, it is not known whether these mechanisms are conserved across vertebrate species. In particular, it is unknown whether mechanisms underlying dopaminergic control of reproduction are shared between mammals and teleost fish. In mammals, dopamine directly inhibits gonadotropin-releasing hormone (GnRH1) hypothalamic neurons, the gatekeepers for activation of the HPG axis. Here, we demonstrate, for the first time in teleost fish, dopaminergic control of GnRH1 neurons via direct dopamine type-2-like receptor (D2R)-mediated inhibition within the hypothalamus. These results suggest that direct dopaminergic control of GnRH1 neurons via interactions in the hypothalamus is not exclusive to tetrapod reproductive control, but is likely conserved across vertebrate species.

Effect of short-term and prolonged stress on the biosynthesis of gonadotropin-releasing hormone (GnRH) and GnRH receptor (GnRHR) in the hypothalamus and GnRHR in the pituitary of ewes during various physiological states

Using an ELISA assay, the levels of GnRH and GnRHR were analysed in the preoptic area (POA), anterior (AH) and ventromedial hypothalamus (VM), stalk/median eminence (SME); and GnRHR in the anterior pituitary gland (AP) of non-breeding and breeding sheep subjected to short-term or prolonged stress. The ELISA study was supplemented with an analysis of plasma LH concentration. Short-term footshock stimulation significantly increased GnRH levels in hypothalamus in both seasons. Prolonged stress elevated or decreased GnRH concentrations in the POA and the VM, respectively during anoestrus, and lowered GnRH amount in the POA-hypothalamus of follicular-phase sheep. An up-regulation of GnRHR levels was noted in both, anoestrous and follicular-phase animals. In the non-breeding period, a prolonged stress procedure increased GnRHR biosynthesis in the VM and decreased it in the SME and AP, while in the breeding time the quantities of GnRHR were significantly lower in the whole hypothalamus. In follicular-phase ewes the fluctuations of GnRH and GnRHR levels under short-term and prolonged stress were reflected in the changes of LH secretion, suggesting the existence of a direct relationship between GnRH and GnRH-R biosynthesis and GnRH/LH release in this period. The study showed that stress was capable of modulating the biosynthesis of GnRH and GnRHR; the pattern of changes was dependent upon the animal's physiological state and on the time course of stressor application. The obtained results indicate that the disturbances of gonadotropin secretion under stress conditions in sheep may be due to a dysfunction of GnRH and GnRHR biosynthetic pathways.

Both Estrogen and Androgen Modify the Response to Activation of Neurokinin-3 and κ-Opioid Receptors in Arcuate Kisspeptin Neurons From Male Mice

Gonadal steroids regulate the pattern of GnRH secretion. Arcuate kisspeptin (kisspeptin, neurokinin B, and dynorphin [KNDy]) neurons may convey steroid feedback to GnRH neurons. KNDy neurons increase action potential firing upon the activation of neurokinin B receptors (neurokinin-3 receptor [NK3R]) and decrease firing upon the activation of dynorphin receptors (κ-opioid receptor [KOR]). In KNDy neurons from intact vs castrated male mice, NK3R-mediated stimulation is attenuated and KOR-mediated inhibition enhanced, suggesting gonadal secretions are involved. Estradiol suppresses spontaneous GnRH neuron firing in male mice, but the mediators of the effects on firing in KNDy neurons are unknown. We hypothesized the same gonadal steroids affecting GnRH firing pattern would regulate KNDy neuron response to NK3R and KOR agonists. To test this possibility, extracellular recordings were made from KNDy neurons in brain slices from intact, untreated castrated or castrated adult male mice treated in vivo with steroid receptor agonists. As observed previously, the stimulation of KNDy neurons by the NK3R agonist senktide was attenuated in intact vs castrated mice and suppression by dynorphin was enhanced. In contrast to observations of steroid effects on the GnRH neuron firing pattern, both estradiol and DHT suppressed senktide-induced KNDy neuron firing and enhanced the inhibition caused by dynorphin. An estrogen receptor-α agonist but not an estrogen receptor-β agonist mimicked the effects of estradiol on NK3R activation. These observations suggest the steroid modulation of responses to activation of NK3R and KOR as mechanisms for negative feedback in KNDy neurons and support the contribution of these neurons to steroid-sensitive elements of a GnRH pulse generator.

Evidence for Changes in Numbers of Synaptic Inputs onto KNDy and GnRH Neurones during the Preovulatory LH Surge in the Ewe

Kisspeptin neurones located in the arcuate nucleus (ARC) and preoptic area (POA) are critical mediators of gonadal steroid feedback onto gonadotrophin-releasing hormone (GnRH) neurones. ARC kisspeptin cells that co-localise neurokinin B (NKB) and dynorphin (Dyn), are collectively referred to as KNDy (Kisspeptin/NKB/Dyn) neurones, and have been shown in mice to also co-express the vesicular glutamate transporter, vGlut2, an established glutamatergic marker. The ARC in rodents has long been known as a site of hormone-induced neuroplasticity, and changes in synaptic inputs to ARC neurones in rodents occur over the oestrous cycle. Based on this evidence, the the present study aimed to examine possible changes across the ovine oestrous cycle in synaptic inputs onto kisspeptin cells in the ARC (KNDy) and POA, and inputs onto GnRH neurones. Gonadal-intact breeding season ewes were perfused using 4% paraformaldehyde during either the luteal or follicular phase of the oestrous cycle, with the latter group killed at the time of the luteinising hormone (LH) surge. Hypothalamic sections were processed for triple-label immunodetection of kisspeptin/vGlut2/synaptophysin or kisspeptin/vGlut2/GnRH. The total numbers of synaptophysin- and vGlut2-positive inputs to ARC KNDy neurones were significantly increased at the time of the LH surge compared to the luteal phase; because these did not contain kisspeptin, they do not arise from KNDy neurones. By contrast to the ARC, the total number of synaptophysin-positive inputs onto POA kisspeptin neurones did not differ between luteal phase and surge animals. The total number of kisspeptin and vGlut2 inputs onto GnRH neurones in the mediobasal hypothalamus (MBH) was also increased during the LH surge, and could be attributed to an increase in the number of KNDy (double-labelled kisspeptin + vGlut2) inputs. Taken together, these results provide novel evidence of synaptic plasticity at the level of inputs onto KNDy and GnRH neurones during the ovine oestrous cycle. Such changes may contribute to the generation of the preovulatory GnRH/LH surge.

Sex differences in the expression of estrogen receptor alpha within noradrenergic neurons in the sheep brain stem

In female sheep, high levels of estrogen exert a positive feedback action on gonadotropin releasing hormone (GnRH) secretion to stimulate a surge in luteinizing hormone (LH) secretion. Part of this action appears to be via brain stem noradrenergic neurons. By contrast, estrogen action in male sheep has a negative feedback action to inhibit GnRH and LH secretion. To investigate whether part of this sex difference is due to differences in estrogen action in the brain stem, we tested the hypothesis that the distribution of estrogen receptor α (ERα) within noradrenergic neurons in the brain stem differs between rams and ewes. To determine the distribution of ERα, we used double-label fluorescence immunohistochemistry for dopamine β-Hydroxylase, as a marker for noradrenergic and adrenergic cells, and ERα. In the ventrolateral medulla (A1 region), most ERα-immunoreactive (-ir) cells were located in the caudal part of the nucleus. Overall, there were more ERα-ir cells in rams than ewes, but the proportion of double-labeled cells was did not differ between sexes. Much greater numbers of ERα-ir cells were found in the nucleus of the solitary tract (A2 region), but <10% were double labeled and there were no sex differences. The majority of ERα-labeled cells in this nucleus was located in the more rostral areas. ERα-labeled cells were found in several rostral brain stem regions but none of these were double labeled and so were not quantified. Because there was no sex difference in the number of ERα-ir cells in the brain stem that were noradrenergic, the sex difference in the action of estrogen on gonadotropin secretion in sheep is unlikely to involve actions on brain stem noradrenergic cells.

Kisspeptin and seasonality of reproduction

Wild and domesticated species display seasonality in reproductive function, controlled predominantly by photoperiod. Seasonal alterations in breeding status are caused by changes in the secretion of gonadotropin-releasing hormone (GnRH) that are mediated by upstream neuronal afferents that regulate the GnRH cells. In particular, kisspeptin appears to play a major role in seasonality of reproduction, transducing the feedback effect of gonadal steroids as well as having an independent (nonsteroid dependent) circannual rhythm. A substantial body of data on this issue has been obtained from studies in sheep and hamsters and this is reviewed here in detail. Kisspeptin function is upregulated during the breeding season in sheep, stimulating reproductive function, but contradictory data are found in Siberian and Syrian hamsters. The relative quiescence of kisspeptin cells in the nonbreeding season can be counteracted by administration of the peptide, leading to activation of reproductive function. Although there is a major role for melatonin in the transduction of photoperiod to the reproductive system, kisspeptin cells do not appear to express the melatonin receptor, so the means by which seasonality changes the level of kisspeptin activity remains unknown.

Evidence that dopamine acts via kisspeptin to hold GnRH pulse frequency in check in anestrous ewes

Recent work has implicated stimulatory kisspeptin neurons in the arcuate nucleus (ARC) as important for seasonal changes in reproductive function in sheep, but earlier studies support a role for inhibitory A15 dopaminergic (DA) neurons in the suppression of GnRH (and LH) pulse frequency in the nonbreeding (anestrous) season. Because A15 neurons project to the ARC, we performed three experiments to test the hypothesis that A15 neurons act via ARC kisspeptin neurons to inhibit LH in anestrus: 1) we used dual immunocytochemistry to determine whether these ARC neurons contain D2 dopamine receptor (D2-R), the receptor responsible for inhibition of LH in anestrus; 2) we tested the ability of local administration of sulpiride, a D2-R antagonist, into the ARC to increase LH secretion in anestrus; and 3) we determined whether an antagonist to the kisspeptin receptor could block the increase in LH secretion induced by sulpiride in anestrus. In experiment 1, 40% of this ARC neuronal subpopulation contained D2-R in breeding season ewes, but this increased to approximately 80% in anestrus. In experiment 2, local microinjection of the two highest doses (10 and 50 nmol) of sulpiride into the ARC significantly increased LH pulse frequency to levels 3 times that seen with vehicle injections. Finally, intracerebroventricular infusion of a kisspeptin receptor antagonist completely blocked the increase in LH pulse frequency induced by systemic administration of sulpiride to anestrous ewes. These results support the hypothesis that DA acts to inhibit GnRH (and LH) secretion in anestrus by suppressing the activity of ARC kisspeptin neurons.

Control of GnRH secretion: one step back

The reproductive system is controlled by gonadotropin releasing hormone (GnRH) secretion from the brain, which is finely modulated by a number of factors including gonadal sex steroids. GnRH cells do not express estrogen receptor α, but feedback is transmitted by neurons that are at least 'one step back' from the GnRH cells. Modulation by season, stress and nutrition are effected by neuronal pathways that converge on the GnRH cells. Kisspeptin and gonadotropin inhibitory hormone (GnIH) neurons are regulators of GnRH secretion, the former being a major conduit for transmission of sex steroid feedback. GnIH cells project to GnRH cells and may play a role in the seasonal changes in reproductive activity in sheep. GnIH also modulates the action of GnRH at the level of the pituitary gonadotrope. This review focuses on the role that kisspeptin and GnIH neurons play, as modulators that are 'one step back' from GnRH neurons.

Neuronal plasticity and seasonal reproduction in sheep

Seasonal reproduction represents a naturally occurring example of functional plasticity in the adult brain as it reflects changes in neuroendocrine pathways controlling gonadotropin-releasing hormone (GnRH) secretion and, in particular, the responsiveness of GnRH neurons to estradiol negative feedback. Structural plasticity within this neural circuitry may, in part, be responsible for seasonal switches in the negative feedback control of GnRH secretion that underlie annual reproductive transitions. We review evidence for structural changes in the circuitry responsible for seasonal inhibition of GnRH secretion in sheep. These include changes in synaptic inputs onto GnRH neurons, as well as onto dopamine neurons in the A15 cell group, a nucleus that plays a key role in estradiol negative feedback. We also present preliminary data suggesting a role for neurotrophins and neurotrophin receptors as an early mechanistic step in the plasticity that accompanies seasonal reproductive transitions in sheep. Finally, we review recent evidence suggesting that kisspeptin cells of the arcuate nucleus constitute a critical intermediary in the control of seasonal reproduction. Although a majority of the data for a role of neuronal plasticity in seasonal reproduction has come from the sheep model, the players and principles are likely to have relevance for reproduction in a wide variety of vertebrates, including humans, and in both health and disease.

Relationship between polysialylated neural cell adhesion molecule and beta-endorphin- or gonadotropin releasing hormone-containing neurons during activation of the gonadotrope axis in short daylength in the ewe

Morphological plasticity has been demonstrated between breeding and anestrous seasons in the ewe hypothalamus, particularly for the gonadotropin-releasing hormone (GnRH) system. We sought to determine the impact of a photoperiodic transition, from long days (LD, 16 h light/24 h) to short days (SD; 8 h light/24 h), on the association between a marker of cerebral plasticity, the polysialylated form of neural cell adhesion molecule (PSA-NCAM), and two diencephalic populations: the GnRH and beta-endorphin (beta-END) neurons, the latter being potent inhibitors of GnRH neuronal activity. We also estimated the number of contacts on GnRH neurons after the passage to SD, using synaptophysin as a marker for synaptic buttons. Those parameters were evaluated in ovariectomized estradiol-replaced ewes using double immunocytochemistry and confocal microscopy at different times after the transition to SD: day 0 (D0), D30, D45, D60 and D112. Luteinizing hormone (LH) secretion was recorded throughout the experiment. High LH levels were observed only at D112. Significantly more PSA-NCAM was found in the GnRH neuron perimeters in the D112 group than in the other groups. This increase was not associated with any change in the number of synaptophysin-immunoreactive contacts on GnRH neurons. The beta-END peri-neuronal space was affected negatively by the transition to SD: the percentage of PSA-NCAM on beta-END neurons decreased between D45 and D112 in the posterior two thirds of the arcuate nucleus (ARC). These results suggest that photoperiod may reorganize cell interactions in different hypothalamic areas, ultimately reactivating GnRH neurons, in our model of ovariectomized-estradiol replaced ewes.

Kisspeptin increases gamma-aminobutyric acidergic and glutamatergic transmission directly to gonadotropin-releasing hormone neurons in an estradiol-dependent manner

GnRH neurons are the final central pathway controlling fertility. Kisspeptin potently activates GnRH release via G protein-coupled receptor 54 (GPR54). GnRH neurons express GPR54, and kisspeptin can act directly; however, GPR54 is broadly expressed, suggesting indirect actions are possible. Transsynaptic mechanisms are involved in estradiol-induced potentiation of GnRH neuron response to kisspeptin. To investigate these mechanisms, separate whole-cell voltage-clamp recordings were performed of gamma-aminobutyric acid (GABA)-ergic and glutamatergic transmission to GnRH neurons in brain slices before and during kisspeptin treatment. To determine whether estradiol alters the effect of kisspeptin on synaptic transmission, mice were ovariectomized and either left with no further treatment (OVX) or treated with estradiol implants (OVX+E). Cells were first studied in the morning when estradiol exerts negative feedback. Kisspeptin increased frequency and amplitude of GABAergic postsynaptic currents (PSCs) in GnRH neurons from OVX+E mice. Blocking action potentials eliminated the effect on frequency, indicating presynaptic actions. Amplitude changes were due to postsynaptic actions. Kisspeptin also increased frequency of glutamatergic excitatory PSCs in cells from OVX+E animals. Kisspeptin did not affect either GABAergic or glutamatergic transmission to GnRH neurons in cells from OVX mice, indicating effects on transmission are estradiol dependent. In contrast to stimulatory effects on GABAergic PSC frequency during negative feedback, kisspeptin had no effect during positive feedback. These data suggest estradiol enables kisspeptin-mediated increases in GABA and glutamate transmission to GnRH neurons. Furthermore, the occlusion of the response during positive feedback implies one consequence of estradiol positive feedback is an increase in transmission to GnRH neurons mediated by endogenous kisspeptin.

Neuroanatomical plasticity in the gonadotropin-releasing hormone system of the ewe: seasonal variation in glutamatergic and gamma-aminobutyric acidergic afferents

Temperate zone animals time the onset of reproductive events to coincide with specific portions of the sidereal year. Although the neural mechanisms involved remain poorly understood, a marked annual variation in the brain's sensitivity to estradiol negative feedback is thought to mediate many of the changes in neuroendocrine hormone secretion, especially that of the gonadotropin-releasing hormone (GnRH) neurons, via neural afferents. The aim of the present study was to determine whether glutamatergic inputs to GnRH neurons in sheep vary seasonally and to expand our previous observations of seasonal changes in gamma-aminobutyric acid (GABA)-ergic inputs. Brains from adult sheep were collected during the breeding season (N = 8) or the nonbreeding season (anestrus; N = 7). Confocal microscopy and optical sectioning were used to quantify the density of labeled VGLUT2 and VGAT immunoreactivity onto GnRH neurons. The results reveal a significantly greater number of VGLUT2-ir inputs to GnRH dendrites during the breeding season vs. the nonbreeding season but no seasonal changes on GnRH cell somas. The number of VGAT-ir terminals onto GnRH dendrites was reduced in the breeding season compared with the nonbreeding season. GnRH neurons were also found to receive dual-phenotype (VGLUT + VGAT) inputs; these varied with season in a manner similar to VGAT inputs. Morphologically, the numbers of branches of proximal dendrites increased significantly in a subset of GnRH neurons located near the midline. Together these results reveal a dynamic seasonal reorganization of identified inputs onto GnRH neurons and lend additional support to the overall hypothesis that seasonal modulation of GnRH neurons involves glutamatergic and GABAergic neural plasticity.

Variation in kisspeptin and RFamide-related peptide (RFRP) expression and terminal connections to gonadotropin-releasing hormone neurons in the brain: a novel medium for seasonal breeding in the sheep

Reproductive activity in sheep is seasonal, being activated by short-day photoperiods and inhibited by long days. During the nonbreeding season, GnRH secretion is reduced by both steroid-independent and steroid-dependent (increased response to estradiol negative feedback) effects of photoperiod. Kisspeptin (also known as metastin) and gonadotropin-inhibitory hormone (GnIH, or RFRP) are two RFamide neuropeptides that appear critical in the regulation of the reproductive neuroendocrine axis. We hypothesized that expression of kisspeptin and/or RFRP underlies the seasonal change in GnRH secretion. We examined kisspeptin and RFRP (protein and mRNA) expression in the brains of ovariectomized (OVX) ewes treated with estradiol (OVX+E) during the nonbreeding and breeding seasons. In OVX+E ewes, greater expression of kisspeptin and Kiss1 mRNA in the arcuate nucleus and lesser expression of RFRP (protein) in the dorsomedial nucleus of the hypothalamus were concurrent with the breeding season. There was also a greater number of kisspeptin terminal contacts onto GnRH neurons and less RFRP-GnRH contacts during the breeding season (compared with the nonbreeding season) in OVX+E ewes. Comparison of OVX and OVX+E ewes in the breeding and nonbreeding season revealed a greater effect of steroid replacement on inhibition of kisspeptin protein and Kiss1 mRNA expression during the nonbreeding season. Overall, we propose that the two RFamide peptides, kisspeptin and RFRP, act in concert, with opposing effects, to regulate the activity of GnRH neurons across the seasons, leading to the annual change in fertility and the cyclical seasonal transition from nonbreeding to breeding season.

Kisspeptin signalling in the brain: Steroid regulation in the rodent and ewe

The Kiss1 gene encodes a family of peptides called kisspeptins, which are the natural ligands for the receptor GPR54. In humans and mice, inactivating mutations of GPR54 results in hypogonadotropic hypogonadism, indicating that kisspeptins play a vital role in the regulation of GnRH secretion. In many species, centrally administered kisspeptins stimulate gonadotrophin secretion in a GnRH-dependant manner. Moreover, virtually all GnRH neurons coexpress GPR54. In the hypothalamus, the vast majority of kisspeptin producing cells also express sex steroid receptors, particularly estrogen receptor alpha. Thus, sex steroids are able to directly regulate the expression of Kiss1 mRNA, implicating kisspeptins as the 'missing link' between sex steroid feedback and GnRH secretion. Kiss1-expressing cells are localised to various regions of the forebrain in rodents, primates and sheep. In the arcuate nucleus (ARC) of the rodent and the ewe, sex steroids inhibit the expression of Kiss1 mRNA, suggesting that the kisspeptin secreting neurons here are the conduit for the negative feedback regulation of GnRH secretion. However, in the rodent anteroventral periventricular nucleus (AVPV), sex steroids induce the expression of Kiss1, implying that these kisspeptin neurons play a role in the positive feedback regulation of GnRH secretion. In sheep, there are no Kiss1 neurons in the AVPV and Kiss1 mRNA expression in the ARC is stimulated immediately prior to the preovulatory GnRH/luteinising hormone surge. Thus, kisspeptin neurons in the ARC of the ewe appear well placed to play a role in the negative and positive feedback regulation of GnRH exerted by sex steroids.

Estradiol induces diurnal shifts in GABA transmission to gonadotropin-releasing hormone neurons to provide a neural signal for ovulation

Ovulation is initiated by a surge of gonadotropin-releasing hormone (GnRH) secretion by the brain. GnRH is normally under negative feedback control by ovarian steroids. During sustained exposure to estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of this steroid switches to positive, inducing the surge. Here, we used an established ovariectomized, estradiol-treated (OVX+E) mouse model exhibiting daily surges to investigate the neurobiological mechanisms underlying this switch. Specifically, we examined changes in GABA transmission to GnRH neurons, which can be excited by GABA(A) receptor activation. Spontaneous GABAergic postsynaptic currents (PSCs) were recorded in GnRH neurons from OVX+E and OVX mice in coronal and sagittal slices. There were no diurnal changes in PSC frequency in cells from OVX mice in either slice orientation. In OVX+E cells in both orientations, PSC frequency was low during negative feedback but increased at surge onset. During the surge peak, this increase subsided in coronal slices but persisted in sagittal slices. Comparison of PSCs before and during tetrodotoxin (TTX) treatment showed TTX decreased PSC frequency in OVX+E cells in sagittal slices, but not coronal slices. This indicates estradiol acts on multiple GABAergic afferent populations to increase transmission through both activity-dependent and -independent mechanisms. Estradiol also increased PSC amplitude during the surge. Estradiol and the diurnal cycle thus interact to induce shifts in both GABA transmission and postsynaptic response that would produce appropriate changes in GnRH neuron firing activity and hormone release.

Physiologic regulation of a tetrodotoxin-sensitive sodium influx that mediates a slow afterdepolarization potential in gonadotropin-releasing hormone neurons: possible implications for the central regulation of fertility

The brain controls fertility through release of gonadotropin-releasing hormone (GnRH), but the mechanisms underlying action potential patterning and GnRH release are not understood. We investigated whether GnRH neurons exhibit afterdepolarizing potentials (ADPs) and whether these are modified by reproductive state. Whole-cell current-clamp recordings of GnRH neurons in brain slices from ovariectomized mice revealed a slow ADP (sADP) after action potentials generated by brief current injection. Generating two or four spikes enhanced sADP amplitude and duration. sADP amplitude was not affected by blocking selected neurotransmitter/neuromodulator receptors, delayed-rectifier potassium channels, calcium-dependent cation channels, or hyperpolarization-activated cation channels but was halved by the calcium channel blocker cadmium and abolished by tetrodotoxin. Cadmium also reduced peak latency. Intrinsic mechanisms underlying the sADP were investigated using voltage-clamp protocols simulating action potential waveforms. A single action potential produced an inward current, which increased after double and quadruple stimulation. Cadmium did not affect current amplitude but reduced peak latency. Pretreatment with blockers of calcium-activated potassium currents (I(KCa)) reproduced this shift and blocked subsequent cadmium-induced changes, suggesting cadmium changes latency indirectly by blocking I(KCa). Tetrodotoxin abolished the inward current, suggesting that it is carried by sodium. In contrast, I(KCa) blockers increased the inward current, indicating that I(KCa) may oppose generation of the sADP. Strong sADPs were suprathreshold, generating repetitive spontaneous firing. I(ADP), sADP, and excitability were enhanced by in vivo estradiol, which triggers a preovulatory surge of GnRH release. Physiological feedback modification of this inward current and resulting sADP may modulate action potential firing and subsequent GnRH release.

Morphological plasticity in the neural circuitry responsible for seasonal breeding in the ewe

An increase in the response of GnRH neurons to estrogen negative feedback is responsible for seasonal anestrus in the ewe, but the underlying neural mechanisms remain largely unknown. Neural plasticity may play an important role because the density of synaptic input to GnRH neurons changes with seasons. Moreover, the transition from breeding to anestrous season requires thyroid hormones, which are also required for neuronal development. In the first experiment, we examined whether the decrease in synapses on GnRH neurons is critical for the transition to anestrus by comparing synaptic input in thyroidectomized and thyroid-intact controls, using electron microscopic analysis. Thyroidectomized ewes remained in the breeding season, but the number of synaptic contacts on their GnRH cells was not different from those in thyroid-intact ewes that were anestrus. The next experiment tested whether there was a seasonal change in morphology of the A15 dopaminergic neurons that mediate estrogen negative feedback during anestrus by analyzing synapsin-positive close contacts onto A15 neurons with confocal microscopy. There was a 2-fold increase in these close contacts onto dendrites of A15 neurons in anestrus and a corresponding increase in the length of A15 dendrites at this time of year. The increase in dendritic length was blocked by thyroidectomy, but this procedure did not significantly affect synaptic input to A15 neurons. These results provide initial evidence that the seasonal change in synapses on GnRH neurons is not sufficient for the transition into anestrus but that plasticity of the A15 dopaminergic neurons mediating estrogen negative feedback may contribute to this seasonal alteration.

Thyroid hormone and γ-aminobutyric acid (GABA) interactions in neuroendocrine systems

Thyroid hormones (THs) have critical roles in brain development and normal brain function in vertebrates. Clinical evidence suggests that some human nervous disorders involving GABA(gamma-aminobutyric acid)-ergic systems are related to thyroid dysfunction (i.e. hyperthyroidism or hypothyroidism). There is experimental evidence from in vivo and in vitro studies on rats and mice indicating that THs have effects on multiple components of the GABA system. These include effects on enzyme activities responsible for synthesis and degradation of GABA, levels of glutamate and GABA, GABA release and reuptake, and GABA(A) receptor expression and function. In developing brain, hypothyroidism generally decreases enzyme activities and GABA levels whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels. Hyperthyroidism does not always have the opposite effect. In vitro studies on adult brain have shown that THs enhance GABA release and inhibit GABA-reuptake by rapid, extranuclear actions, suggesting that presence of THs in the synapse could prolong the action of GABA after release. There are conflicting results on effects of long term changes in TH levels on GABA reuptake. Increasing and decreasing circulating TH levels experimentally in vivo alter density of GABA(A) receptor-binding sites for GABA and benzodiazepines in brain, but results vary from study to study, which may reflect important regional differences in the brain. There is substantial evidence that THs also have an extranuclear effect to inhibit GABA-stimulated Cl(-) currents by a non-competitive mechanism in vitro. The thyroid gland exhibits GABA transport mechanisms as well as enzyme activities for GABA synthesis and degradation, all of which are sensitive to thyroidal state. In rats and humans, GABA inhibits thyroid stimulating hormone (TSH) release from the pituitary, possibly by action directly on the pituitary or on hypothalamic thyrotropin-releasing hormone neurons. In mice, GABA inhibits TSH-stimulated TH release from the thyroid gland. Taken together, these studies provide strong support for the hypothesis that there is reciprocal regulation of the thyroid and GABA systems in vertebrates.

The role of noradrenaline in the generation of the preovulatory LH surge in the ewe

Increasing plasma estrogen (E) levels during the follicular phase of the estrous cycle trigger the pre-ovulatory surge of gonadotropin-releasing hormone (GnRH)/LH. Noradrenaline (NA)-producing cells of the brain stem are involved in regulating GnRH cells and project to the preoptic area (POA) and bed nucleus of stria terminalis (BnST). Input to GnRH cells may be direct or indirect, via relay neurons in the POA/BnST. To investigate this, we ascertained whether an alpha(1)-adrenergic antagonist would block/delay the LH surge in ovariectomised (OVX), E-treated ewes. E benzoate (EB) (50microg) was injected (i.m.) and Doxazosin (100nmol/h) or vehicle was infused into the third ventricle 2-26h after EB injection. Doxazosin reduced the magnitude of the LH surge, but did not affect timing. To determine if NA is released in the POA/BnST of cyclic ewes, we immunostained dopamine-beta-hydroxylase (DBH) in terminal fields. Reduced numbers of varicosities staining for DBH indicates release of NA. The number of varicosities immunostained for DBH was reduced in the dorsal and lateral BnST during the follicular phase and during the preovulatory LH surge compared to the luteal phase. These data suggest that noradrenergic mechanisms are involved in generation of the GnRH/LH surge via projections to the BnST and relay to GnRH cells. Since Doxasozin reduced the magnitude of the LH surge in the E-treated OVX ewe, and release of NA in cyclic ewes occurred during the follicular phase of the estrous cycle, we speculate that NA is a permissive factor in surge generation. Thus, increased noradrenergic activity is not a trigger mechanism for initiation of the surge.

Endogenous activation of metabotropic glutamate receptors modulates GABAergic transmission to gonadotropin-releasing hormone neurons and alters their firing rate: a possible local feedback circuit

Gonadotropin-releasing hormone (GnRH) neurons are the primary central regulators of fertility, receiving input from GABAergic afferents via GABA(A) receptors. We tested whether metabotropic glutamate receptors (mGluRs) regulate GABA transmission to GnRH neurons and GnRH neuronal firing pattern. Whole-cell recordings were performed under conditions isolating ionotropic GABA postsynaptic currents (PSCs) in brain slices. The broad-spectrum mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) decreased the frequency of GABA(A)-mediated spontaneous PSCs in a reversible manner. Amplitude and kinetics were not altered, suggesting that afferent GABA neurons are the primary targets. TTX eliminated the effects of ACPD in most tested neurons. Group II [2-(2,3-dicarboxycyclopropyl) glycine] and III (L-AP-4) mGluR agonists mediated this response; a group I agonist (3,5-dihydroxyphenylglycine) was not effective. The broad-spectrum antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) and/or (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) (group III antagonist) enhanced spontaneous PSC frequency, particularly when initial frequency was low, suggesting that endogenous activation of mGluRs regulates GABA transmission to GnRH neurons. Extracellular recordings were used to evaluate GnRH neuron firing rate within the network. ACPD reduced firing rate, and MCPG plus CPPG had an opposite effect, indicating that mGluRs help control excitability of the GnRH network. GnRH neurons express vesicular glutamate transporters, suggesting they may corelease this transmitter. Simulation of firing activity in a GnRH neuron decreased PSC frequency in that cell, an effect blocked by antagonism of mGluRs but not GnRH receptors. These results demonstrate an inhibition of GABAergic inputs to GnRH neurons by mGluRs via a presynaptic mechanism. Through this mechanism, local glutamate milieu, possibly contributed by GnRH neurons themselves, plays an important role in modulating GnRH release and the central regulation of fertility.

Endogenous gamma-aminobutyric acid can excite gonadotropin-releasing hormone neurons

gamma-Aminobutyric acid (GABA) provides a major synaptic input to GnRH neurons. GnRH neurons maintain high intracellular chloride levels and respond to exogenous GABA with depolarization and action potential firing. We examined the role of synaptic GABA type A receptor (GABA(A)R) activation on the firing activity of GnRH neurons. Targeted extracellular recordings were used to detect firing activity of GnRH neurons in brain slices from adult female mice. Because the brain slice preparation preserves both glutamatergic and GABAergic neuronal networks, the effects of GABA(A)Rs on GnRH neurons were isolated by blocking ionotropic glutamatergic receptors (iGluR). With iGluR blocked, many GnRH neurons remained spontaneously active. Consistent with an excitatory role for GABA, subsequent blockade of GABA(A)Rs suppressed the firing rate in active cells from diestrous females by approximately 40% (P < 0.05; n = 10). GABA(A)R blockade did not affect inactive cells (n = 7), indicating that GABA(A)R-mediated inhibition was not responsible for the lack of firing. In prenatally androgenized females, GnRH neurons exhibit larger, more frequent GABAergic postsynaptic currents than control females. Most cells from prenatally androgenized animals fired spontaneously, and the firing rate was suppressed approximately 80% after GABA(A)R blockade (P < 0.01; n = 8). Blocking GABA(A)R without blocking iGluRs increased the firing rate in GnRH neurons from diestrous females (P < 0.05; n = 6), perhaps attributable to hyperexcitability within the slice network. Our results indicate that GABAergic inputs help generate a portion of action potentials in GnRH neurons; this fraction depends on the level of GABA transmission and postsynaptic responsiveness. The complexities of the GnRH neuron response to GABA make this a potentially critical integration point for central regulation of fertility.

Reduction in adiposity affects the extent of afferent projections to growth hormone-releasing hormone and somatostatin neurons and the degree of colocalization of neuropeptides in growth hormone-releasing hormone and somatostatin cells of the ovine hypothalamus

Various neuropeptides and neurotransmitters affect GH secretion by acting on GHRH and somatostatin (SRIF) cells. GH secretion is also affected by alteration in adiposity, which could be via modulation of GHRH and SRIF cells. We quantified colocalization of neuropeptides in GHRH and SRIF cells and afferent projections to these cells in lean (food restricted) and normally fed sheep (n=4/group). The number of GHRH-immunoreactive (IR) cells in the arcuate nucleus was higher in lean animals, but the number of SRIF-IR cells in the periventricular nucleus was similar in the two groups. A subpopulation of GHRH-IR cells colocalized neuropeptide Y in lean animals, but this was not seen in normally fed animals. GHRH/galanin (GAL) colocalization was higher in lean animals with no difference in numbers of GHRH/tyrosine hydroxylase or GHRH/GAL-like peptide cells. SRIF/enkephalin colocalization was lower in lean animals. The percentage of GHRH neurons receiving SRIF input was similar in lean and normally fed animals, but more GHRH cells received input from enkephalin afferents in normally fed animals. The percentage of SRIF cells receiving GHRH, neuropeptide Y, GAL, and orexin afferents was higher in lean animals. These findings provide an anatomical evidence of central mechanism(s) by which appetite-regulating peptides and dopamine could regulate GH secretion. Increased input to SRIF cells in lean animals may be inhibitory and permissive of increased GH. The appearance of NPY in GHRH cells of lean animals may be a mechanism for regulation of increasing GH secretion with reduced adiposity.

Synthesis and secretion of GnRH

Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells.

Sex steroid regulation of brain glutamic acid decarboxylase (GAD) mRNA is season-dependent and sexually dimorphic in the goldfish Carassius auratus

GABA, the major inhibitory neurotransmitter of the vertebrate brain, has been shown to play an important role in vertebrate reproduction by regulating LH release and sexual behavior. We have studied the expression of the GABA-synthesizing enzyme, glutamic acid decarboxylase (GAD), in goldfish throughout the reproductive cycle in May (mature), November (early gonadal recrudescence) and February (late gonadal recrudescence) and in response to implanted sex steroids. Levels of GAD67 and GAD65 mRNA levels in the hypothalamus of both males and females were highest in the early stages of gonadal recrudescence. In the telencephalon, a different seasonal pattern of GAD expression was evident. The telencephalic expression GAD67, GAD65 and a novel isoform, GAD3, were highest in sexually mature fish in May. Five-day intraperitoneal implantation of gonad-intact fish with testosterone (T), estradiol (E2) or progesterone (P4) did not affect GAD expression in November and February. This is in contrast to results in May when sex differences in steroid responsiveness were evident. Progesterone decreased hypothalamic GAD67 and GAD65 in females and was without effect in males. All other treatments did not alter GAD67, GAD65 or GAD3 expression in the hypothalamus. Both T and P4 decreased GAD67 and GAD65 levels in the telencephalon of male goldfish but had no effect in females. Serum sex steroid levels in control and implanted mature males and females in May were similar so it is unlikely that sex differences in the GAD responses were a result of differences in serum sex steroid levels. These contrasting effects of sex steroids on males and females suggest important sex differences in the regulation of the GADs in sexually mature goldfish.

Evidence that projections from the bed nucleus of the stria terminalis and from the lateral and medial regions of the preoptic area provide input to gonadotropin releasing hormone (GNRH) neurons in the female sheep brain

The arcuate nucleus/ventromedial hypothalamic nucleus (ARC/VMH) region is thought to relay estrogen feedback signals to gonadotropin-releasing hormone (GnRH) cells in the sheep brain. This region sends major projections to the lateral preoptic area (lPOA), ventral bed nucleus of the stria terminals (vBnST) and the ventro-caudal division of the median preoptic nucleus (vcMePON) with little direct input to GnRH cell bodies, suggesting interneuronal relay to GnRH neurons. The brain stem also provides input to the POA. The present study aimed to identify possible relay circuits in the POA and BnST to GnRH neurons. Biotinylated dextran amine (BDA) was injected into lPOA (n=6), vBnST (n=2), vcMePON (n=3) and periventricular nucleus (PeriV; n=1) of ewes for anterograde tracing. GnRH immunoreactive (IR) perikarya appearing to receive input from BDA-containing varicosities were identified by fluorescence microscopy, with further analysis by confocal microscopy. When BDA was injected into rostral and caudal regions of lPOA (n=3), no tracer-filled varicose fibers were found in contact with GnRH-IR perikarya. Injections into the center of the lPOA (n=3) indicated direct projections to GnRH-IR cells. Injections into the vBnST, vcMePON and PeriV indicated that cells of these regions also provide input to GnRH cells. BDA-containing varicosities found in the MPOA were immunoreactive for NPY or were GABAergic or glutamatergic when the tracer was injected into vBnST and lPOA, but not when injections were placed in the vcMePON. With injection into the PeriV, tracer-filled varicosities in the MPOA were not immunoreactive for somatostatin or enkephalin. Injection of FluoroGold into ventral POA retrogradely labeled cells in the above mentioned areas, but few were also immunoreactive for estrogen receptor-alpha. Thus, cells of the vBnST, lPOA, vcMePON and PeriV project to GnRH neurons. These cells may provide an interneuronal route to GnRH neurons from the ARC/VMH, the brain stem and other regions of the brain.

Noradrenergic regulation of hypothalamic cells that produce growth hormone-releasing hormone and somatostatin and the effect of altered adiposity in sheep

The growth hormone (GH) axis is sensitive to alteration in body weight and there is evidence that central noradrenergic systems regulate neurones that produce growth hormone-releasing hormone (GHRH) and somatostatin (SRIF). This study reports semiquantitative estimates of the noradrenergic input to neuroendocrine GHRH and SRIF neurones in the sheep of different body weights. We also studied the effects of altered body weight on expression of dopamine beta-hydroxylase (DBH), the enzyme that produces noradrenalin from dopamine. Ovariectomised ewes were made Lean (39.6 +/- 2.6 kg; Mean +/- SEM) by dietary restriction, whereas Normally Fed animals (61.2 +/- 0.8 kg) were maintained on a regular diet. Brains were perfused for immunohistochemistry and in situ hybridisation. The Mean +/- SEM number of GHRH-immunoreactive (-IR) cells was lower in Normally Fed (65 +/- 7) than in Lean (115 +/- 14) animals, whereas the number of SRIF-IR cells was similar in the two groups (Normally Fed, 196 +/- 17; Lean 230 +/- 21). Confocal microscopic analysis revealed that the percentage of GHRH-IR cells (Normally Fed 36 +/- 1.5% versus Lean 32 +/- 4.6%) and percentage of SRIF-IR cells (Normally Fed 30 +/- 40.4% versus Lean 32 +/- 2.3%) contacted by noradrenergic fibres did not change with body weight. FluoroGold retrograde tracer injections confirmed that noradrenergic projections to the arcuate nucleus are from ventrolateral medulla and noradrenergic projections to periventricular nucleus arise from the ventrolateral medulla, nucleus of solitary tract, locus coeruleus (LC) and the parabrachial nucleus (PBN). DBH expressing cells were identified using immunohistochemistry and in situ hybridisation and the level of expression (silver grains/cell) quantified by image analysis. The number of DBH cells was similar in Normally Fed and Lean animals, but the level of expression/cell was lower (P < 0.02) in the PBN and LC of Lean animals. These results provide an anatomical basis for the noradrenergic regulation of GHRH and SRIF cells and GH secretion. Altered activity or noradrenergic neurones in the PBN and LC that occur with reduced body weight may be relevant to the control of GH axis.

Progesterone-receptive beta-endorphin and dynorphin B neurons in the arcuate nucleus project to regions of high gonadotropin-releasing hormone neuron density in the ovine preoptic area

Progesterone inhibits gonadotropin-releasing hormone (GnRH) secretion through interneuronal systems located in the mediobasal hypothalamus in ewes. Endogenous opioid peptides are implicated in this inhibition of GnRH secretion. The distributions of endogenous opioid peptides are known to overlap with progesterone receptors (PR) in the arcuate nucleus. We investigated whether PR is expressed by beta-endorphin and dynorphin B neurons in the arcuate nucleus and if a subset of double-labeled cells projects to the preoptic area where most GnRH neurons are detected. Injection of a retrograde tracer, Fluorogold, into the rostral preoptic area was performed in ovariectomized ewes pretreated with estrogen and progesterone. Brain sections were processed using double immunocytochemistry. Only brains of ewes with an injection site encompassing at least 80 GnRH neurons were processed for PR and then either beta-endorphin or dynorphin B immunocytochemistry. Antigen retrieval is essential for PR detection but causes Fluorogold to fade. Thus, quantitative analysis was performed on photographs taken before and after antigen retrieval. We found that 25-30% of PR-containing neurons, 20% of beta-endorphin cells and 22% of dynorphin B neurons in the arcuate nucleus project toward the preoptic area. From the PR/beta-endorphin double-labeled cells that represent 25 and 36% of PR and beta-endorphin cells, respectively, 35% were labeled with Fluorogold. From the PR/dynorphin B double-labeled cells that account for 39 and 62% of PR and dynorphin B neurons, respectively, 26% contained Fluorogold. These data strongly support the hypothesis that progesterone acts in the arcuate nucleus through beta-endorphin and dynorphin B neurons to affect preoptic area GnRH neurons.

Progesterone-receptive dopaminergic and neuropeptide Y neurons project from the arcuate nucleus to gonadotropin-releasing hormone-rich regions of the ovine preoptic area

Progesterone inhibits gonadotropin-releasing hormone (GnRH) secretion in sheep through an interneuronal system located in the mediobasal hypothalamus. This study focused on known inhibitors of GnRH secretion in sheep, dopamine and neuropeptide Y (NPY). As the distributions of tyrosine hydroxylase (TH)- and NPY-immunoreactive neurons overlap with progesterone receptors (PR) in the arcuate nucleus, we hypothesized that, if these neurons mediate, at least partially, the inhibitory feedback signal of progesterone, then they should co-express PRs. Fluorogold (FG), a retrograde tracer, was injected into the preoptic area of ovariectomized ewes pretreated with estrogen and progesterone. When the FG injection site encompassed at least 80 GnRH neurons, sections from the arcuate nucleus were processed using dual immunocytochemistry for PR and either TH or NPY. We found that 30% of PR-immunoreactive, 12% of TH-containing and 21% of NPY-synthesizing neurons project toward this GnRH-rich region. Of the PR/TH dual-labeled cells, which represent 21% of PR and 31% of TH cells, respectively, 22% displayed FG labeling. Of the PR/NPY neurons, which account for 19% of PR and 67% of NPY neurons, respectively, 26% were FG fluorescent. This study suggests that subsets of arcuate nucleus dopaminergic and NPY neurons may transduce, at least in part, the progesterone-mediated inhibition of GnRH secretion.

GABAergic integration of progesterone and androgen feedback to gonadotropin-releasing hormone neurons

Steroid feedback regulates GnRH secretion and previous work has implicated gamma-aminobutyric acid (GABA)ergic neurons as a mediator of these effects. We examined GABAergic postsynaptic currents (PSCs) in green fluorescent protein-identified GnRH neurons from mice exposed to different steroid milieus in vivo. Adult mice were ovariectomized and treated with estradiol (OVX+E, controls) or E plus progesterone (P, OVX+E+P). P decreased PSC frequency, a presynaptic effect, and PSC size, which could be via pre- and/or postsynaptic mechanisms. In contrast, dihydrotestosterone (DHT, OVX+E+DHT) increased both GABAergic PSC frequency and size in GnRH neurons. Tetrodotoxin (TTX), which eliminates action-potential-dependent presynaptic effects, did not alter frequency, suggesting DHT may have increased PSC frequency by increasing connectivity between GABAergic and GnRH neurons. TTX reduced PSC size below control values, indicating DHT may augment presynaptic GABA release but inhibits the postsynaptic GnRH neuron response. In mice treated with both P and DHT (OVX+E+P+DHT), PSC frequency and size were similar to controls, suggesting these steroids counteract one another. These results demonstrate GABAergic neurons participate in integrating and conveying steroid feedback to GnRH neurons, defining a potential central mechanism for steroid regulation of GnRH neurons during the reproductive cycle, and providing one possible mechanism for increased activity of these cells in hyperandrogenic females.

Gamma-aminobutyric acid neurons integrate and rapidly transmit permissive and inhibitory metabolic cues to gonadotropin-releasing hormone neurons

Negative energy balance inhibits fertility by decreasing GnRH release; however, the mechanisms are not well understood. GnRH neurons can be excited by activation of gamma-aminobutyric acid (GABA)(A) receptors, and GABAergic neurons provide a major synaptic input. We hypothesized that permissive metabolic signals mediated by leptin and inhibitory signals conveyed by neuropeptide Y (NPY) and opiates rapidly alter GABA(A) receptor-mediated drive to GnRH neurons. In fed and fasted female mice, GABAergic postsynaptic currents (PSCs) were recorded from GnRH neurons before and after in vitro treatment with leptin, NPY, or met-enkephalin. Leptin increased PSC frequency in fed and fasted mice, indicating that it increased presynaptic activity. Leptin also increased PSC size. Inhibiting leptin receptor signaling pathways within GnRH neurons abolished the latter effect, indicating a direct action on these cells. In fed, but not fasted, mice, NPY and met-enkephalin decreased PSC frequency in an antagonist-reversible manner, but did not alter PSC size. NPY-1 receptor antagonists alone increased frequency in fed and fasted mice, as did opiate receptor blockade in fasted animals, suggesting that endogenous NPY and opiates modulate GABAergic drive to GnRH neurons. These data suggest that GABAergic afferents integrate metabolic signals for delivery to GnRH neurons. Decreased sensitivity to NPY and opiates in fasted mice indicate that these peptides send physiologically relevant signals regarding energy balance to GnRH neurons.

Evidence for vesicular glutamate transporter synapses onto gonadotropin-releasing hormone and other neurons in the rat medial preoptic area

The medial preoptic area is a key structure in the control of reproduction. Several data suggest that excitatory amino acids are involved in the regulation of this function and the major site of this action is the medial preoptic region. Data concerning the neuromorphology of the glutamatergic innervation of the medial preoptic area are fragmentary. The present investigations were focused on: (i) the morphology of the vesicular glutamate transporter 1 (VGluT1)- and vesicular glutamate transporter 2 (VGluT2)-immunoreactive nerve terminals, which are considered to be specific to presumed glutamatergic neuronal elements, in the medial preoptic area of rat; and (ii) the relationship between these glutamate transporter-positive endings and the gonadotropin-releasing hormone (GnRH) neurons in the region. Single- and double-label immunocytochemistry was used at the light and electron microscopic level. There was a weak to moderate density of VGluT1- and a moderate to intense density of VGluT2-immunoreactive elements in the medial preoptic area. Electron microscopy revealed that both VGluT1- and VGluT2-immunoreactive boutons made asymmetric type synaptic contacts with unlabelled neurons. VGluT2-labelled, but not VGluT1-labelled, axon terminals established asymmetric synaptic contacts on GnRH-immunostained neurons, mainly on their dendrites. The present findings are the first electron microscopic examinations on the glutamatergic innervation of the rat medial preoptic area. They provide direct neuromorphological evidence for the existence of direct glutamatergic innervation of GnRH and other neurons in the rat medial preoptic area.

Evidence for estrogenic regulation of gonadotropin-releasing hormone neurons by glutamatergic neurons in the ewe brain: An immunohistochemical study using an antibody against vesicular glutamate transporter-2

Gonadotropin-releasing hormone (GnRH) secretion is controlled by various factors, including the excitatory neurotransmitter glutamate. Estrogen (E) regulates GnRH secretion by means of E-responsive cells in the brain that relay the feedback effects to the preoptic area (POA). We used an antibody to vesicular glutamate transporter 2 (VGluT2) to label glutamatergic neurons in the areas of the ewe brain that control GnRH secretion. VGluT2-immunoreactive cells were observed in the arcuate nucleus (ARC)/ventromedial hypothalamic nucleus (VMH) complex, POA, bed nucleus of stria terminalis (BnST), and A1 and A2 cell groups in the brainstem. In three ewes, E receptor-alpha was detected in 52-61% of glutamatergic neurons in ARC/VMH, 37-52% of neurons in the POA, and 37-58% of neurons in the BnST. E injection (i.m. or i.v.) increased the percentage of glutamatergic cells that expressed Fos protein in the ARC (P < 0.01 and P < 0.001, respectively). In six ewes, injection of the retrograde tracer Fluoro-Gold into the POA labeled cells in the ARC and 6-29% of these were also VGluT2-immunoreactive. Double-labeling of varicosities in the POA showed colocalization of VGluT2 in 12.5 +/- 3% of dopamine beta-hydroxylase-immunoreactive terminals, indicating that a subset of glutamatergic inputs could arise from brainstem noradrenergic neurons cells. In the POA, 60% of GnRH neurons had close appositions that were VGluT2-immunoreactive. We conclude that E-responsive glutamatergic neurons arising from the brainstem, the BnST, and ARC/VMH provide input to the POA and may be involved in the regulation of GnRH secretion.

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.