Photoperiod regulates the LH response to central glutamatergic stimulation in the male Syrian hamster

Time course of photo-induced Egr-1 expression in the hypothalamus of a seasonally breeding songbird

Many seasonally-breeding species use daylength to time reproduction. Light-induced release of progonadal hormones involves a complex cascade of responses both inside and outside the brain. In this study, we used induction of early growth response 1 (Egr-1), the protein product of an immediate early gene, to evaluate the time course of such responses in male white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. Induction of Egr-1 in the pars tuberalis began ∼11 h after dawn. This response was followed ∼6 h later by dramatic induction in the tuberal hypothalamus, including in the ependymal cells lining the third ventricle. At approximately the same time, Egr-1 was induced in dopaminergic and vasoactive intestinal peptide neurons in the tuberal hypothalamus and in dopaminergic neurons of the premammillary nucleus. We noted no induction in gonadotropin-releasing hormone (GnRH) neurons until 2 h after dawn the following morning. Overall, our results indicate that Egr-1 responses in GnRH neurons occur rather late during photostimulation, compared with responses in other cell populations, and that such induction may reflect new synthesis related to GnRH depletion rather than stimulation by light cues.

Gonadotrophin-releasing hormone neurons in a photoperiodic songbird express fos and egr-1 protein after a single long day

Birds use a variety of environmental cues, such as day length, temperature and social interactions, to time reproductive efforts. For most seasonally breeding birds, day length is the most important cue and takes precedence over all others. Experimental manipulation of day length has shown that, in a number of galliformes and passeriformes, exposure to a single long day induces a rise in plasma luteinising hormone (LH). The mechanisms underlying this response are only beginning to be understood. In Japanese quail and Zonotrichia sparrows, one long day causes striking up-regulation of the protein products of immediate early genes (IEGs) in the mediobasal hypothalamus, near gonadotrophin-releasing hormone (GnRH) axons and terminals. Photoperiodic induction of the same proteins in the GnRH somata themselves, however, has not been described in these species. In the present study, we used immunohistochemistry to assay the induction of two IEGs, Fos and Egr-1, in the GnRH somata of male and female white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. We found that immunoreactivity for both proteins increased in a subset of the GnRH neurones of the septo-preoptic area by the morning after the long day. Photo-induced expression of Egr-1 or Fos protein in GnRH neurones was limited to a population of cells in the medial preoptic area. Males showed significantly greater induction of both proteins in this population of GnRH neurones than did females, which is consistent with the hypothesis that males may be more sensitive to photic cues. Overall, the results obtained suggest that photostimulation stimulates new protein synthesis in GnRH neurones on a relatively rapid time scale. Further research is required to determine whether the GnRH somata are themselves integrating photic cues, or whether they are responding rapidly to an increased demand for GnRH synthesis.

Estradiol negative feedback regulation by glutamatergic afferents to A15 dopaminergic neurons: variation with season

It is now clear that seasonal breeding in ewes is due to an increase in response to estradiol (E(2)) negative feedback in the nonbreeding season (anestrus) that is mediated by the A15 group of dopaminergic (DA) neurons. Because A15 cells do not contain estrogen receptors, we have postulated the presence of estrogen-responsive afferents and recently reported evidence that input from neurons containing gamma-aminobutyric acid (GABA) contribute to the control of A15 activity by E(2). However, GABAergic afferents account for only a fraction of A15 synaptic input and do not appear to vary with season. We therefore investigated the possible role of stimulatory glutamatergic input to A15 neurons. In experiments 1 and 2, local administration into the A15 of either a N-methyl-D-aspartate (NMDA) receptor or a kainate/alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor antagonist stimulated episodic LH secretion in a dose-dependent manner in ovary-intact anestrous ewes. In experiment 3, we examined the number of glutamatergic close contacts onto A15 neurons using dual immunocytochemistry in tissue from E(2)-treated ovariectomized anestrous and breeding season ewes. All A15 DA neurons were contacted by glutamatergic vesicles, and the number of close contacts was significantly higher in anestrus than the breeding season. Finally, using a triple-label immunocytochemistry procedure, we did not observe any colocalization of markers for GABA and glutamate in vesicles contacting A15 neurons. These results support the hypothesis that glutamatergic afferents actively stimulate A15 DA neurons in ovary-intact anestrous ewes and raise the possibility that alterations in this input may contribute to increased A15 neural activity during anestrus.

Kisspepeptin-GPR54 signaling in the neuroendocrine reproductive axis

Kisspeptins, which are products of the Kiss1 gene, and their receptor, GPR54, have emerged as key players in the regulation of gonadotropin-releasing hormone (GnRH) secretion. Mutations or targeted deletions of GPR54 produce isolated hypogonadotropic hypogonadism in humans and mice, indicating that signaling through this receptor is a prerequisite for sexual maturation. Centrally administered kisspeptins stimulate GnRH and gonadotropin secretion in prepubertal and adult animals. Kisspeptin-expressing neurons are direct targets for the negative and positive feedback actions of sex steroids, which differentially regulate the expression of KiSS-1 mRNA in various regions of the forebrain. This review highlights what is currently known about kisspeptin-GPR54 signaling in the regulation of the neuroendocrine reproductive axis.

Expression of glutamate receptor subunits in the hypothalamus of the female rat during the afternoon of the proestrous luteinizing hormone surge and effects of antiprogestin treatment and aging

The excitatory transmitter, glutamate has been implicated in the control of reproduction, hormone secretion and neuroendocrine regulation. The present study examined whether the hypothalamic expression of three key ionotropic glutamate receptor subunits (NMDAR1, GluR1 and GluR6) fluctuates significantly on proestrus in the rat, and whether treatment with the antiprogestin, RU486 affected glutamate receptor subunit expression. The studies revealed that NMDAR1, GluR1 and GluR6 mRNA levels in the mediobasal hypothalamus (MBH) and preoptic area (POA) fluctuate little throughout the day of proestrus. However, treatment with the antiprogestin, RU486 induced a significant elevation of GluR6 mRNA levels at 14.00 and 16.00 h on proestrus in the MBH, suggesting that endogenous progesterone (P4) may act to inhibit hypothalamic GluR6 levels. In support of this suggestion, exogenous P4 treatment to estrogen (E2)-primed ovariectomized (ovx) rats significantly suppressed GluR6 mRNA levels in the afternoon (12.00-16.00 h) in the MBH, and at 12.00 h in the POA, which preceded LH surge induction. Likewise, temporal examination of hypothalamic GluR6 protein levels in E2 + P4-treated young and middle-aged ovx rats revealed an early elevation from 12.00 to 14.00 h, which was followed by a fall from 16.00 to 20.00 h. The early elevation of GluR6 protein levels was most pronounced in the POA of the young rat, and this elevation was markedly attenuated in the middle-aged rat. As a whole, the studies suggest that glutamate receptor expression fluctuates little on proestrus in the hypothalamus, but that expression of the kainate GluR6 receptor subunit may be modulated by progesterone and aging.

A role for kisspeptins in the regulation of gonadotropin secretion in the mouse

Kisspeptins are products of the KiSS-1 gene, which bind to a G protein-coupled receptor known as GPR54. Mutations or targeted disruptions in the GPR54 gene cause hypogonadotropic hypogonadism in humans and mice, suggesting that kisspeptin signaling may be important for the regulation of gonadotropin secretion. To examine the effects of kisspeptin-54 (metastin) and kisspeptin-10 (the biologically active C-terminal decapeptide) on gonadotropin secretion in the mouse, we administered the kisspeptins directly into the lateral cerebral ventricle of the brain and demonstrated that both peptides stimulate LH secretion. Further characterization of kisspeptin-54 demonstrated that it stimulated both LH and FSH secretion, at doses as low as 1 fmol; moreover, this effect was shown to be blocked by pretreatment with acyline, a potent GnRH antagonist. To learn more about the functional anatomy of kisspeptins, we mapped the distribution of KiSS-1 mRNA in the hypothalamus. We observed that KiSS-1 mRNA is expressed in areas of the hypothalamus implicated in the neuroendocrine regulation of gonadotropin secretion, including the anteroventral periventricular nucleus, the periventricular nucleus, and the arcuate nucleus. We conclude that kisspeptin-GPR54 signaling may be part of the hypothalamic circuitry that governs the hypothalamic secretion of GnRH.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

AMPA glutamate receptor subunits in the guinea pig hypothalamus: distribution and colocalization with progesterone receptor

Excitatory amino acids (EAAs), particularly glutamate, have been implicated in the control of luteinizing hormone (LH) secretion through facilitation of gonadotropin-releasing hormone release. The effects of EAAs are mediated by means of ionotropic glutamate receptors, which are divided into N-methyl-D-aspartate (NMDA) and non-NMDA (kainate and AMPA) subtypes. Moreover, ovarian steroids are responsible for inducing the preovulatory surge of LH and are involved in the actions of EAAs on LH release. Progesterone is directly involved in the potentiating effect of ovarian steroids on the stimulating effect of AMPA neurotransmission on gonadotropin secretion. To broaden our understanding of the role of hypothalamic AMPA receptors in the steroid-induced LH surge, we determined the cellular localization of AMPA receptors in the hypothalamus of guinea pigs by using antibodies that recognize the GluR1, GluR2, GluR2/3, or GluR4 subunits, and then we examined the neuroanatomic relationships between these receptors and the progesterone receptor (PR). Different patterns of immunostaining within the preoptic area and hypothalamus were evident with the antibodies to the four subunits with marked contrasts between moderate staining for GluR1, intensely stained structures for GluR2 and GluR2/3, and little specific staining for GluR4. Immunoreactive (IR) neurons were visualized in many regions, including the two regions known to contain a dense population of estradiol-induced PR-IR cells: the preoptic periventricular and ventrolateral hypothalamic nuclei. Approximately 60% of GluR1-IR and 39% of GluR2-IR cells in the preoptic region possessed PR, whereas 46% of GluR1-IR and 54% of GluR2-IR cells in the ventrolateral nucleus expressed PR. These neuroanatomic results suggest that the coordinated actions of progesterone and glutamatergic inputs on mammalian reproductive functions are integrated at the cellular level.

Changes in basal hypothalamic chicken gonadotropin-releasing hormone-I and vasoactive intestinal polypeptide associated with a photo-induced cycle in gonadal maturation and prolactin secretion in intact and thyroidectomized starlings (Sturnus vulgaris)

Chicken gonadotropin-releasing hormone-I (GnRH-I) and the avian prolactin-releasing hormone, vasoactive intestinal polypeptide (VIP), were measured in the basal hypothalamus in male starlings during photo-induced gonadal growth and the subsequent development and maintenance of reproductive photorefractoriness. Comparisons were made with thyroidectomized birds, which maintain breeding condition irrespective of changes in photoperiod. In intact birds, basal hypothalamic GnRH-I increased four-fold after photostimulation and then decreased 115-fold over 12 weeks to values characteristic of long-term photorefractoriness. Pituitary and plasma prolactin increased after photostimulation, reaching peak values when the testes were regressing, and returned to low values in long-term photorefractory birds. Basal hypothalamic VIP did not change after photostimulation in intact birds. In photostimulated thyroidectomized birds, values for basal hypothalamic GnRH-I and VIP, and for pituitary and plasma prolactin, remained no different to those of nonphotostimulated intact birds. These observations confirm that reproductive photorefractoriness is related to a decrease in hypothalamic GnRH-I. However, photorefractoriness in terms of prolactin secretion is not similarly related to a decrease in basal hypothalamic VIP. The mechanisms responsible for the decrease in prolactin in long-term photorefractory birds and for the total lack of photoperiodic responses in thyroidectomized birds remain unresolved.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.

Effects of administration of N-methyl-D,L-aspartate (NMA) on gonadotropin secretion in untreated and steroid-treated ovariectomized mares during the breeding season and in intact and ovariectomized mares during anestrus

The purpose of this experiment was to investigate whether N-methyl-D,L-aspartate stimulated gonadotropin secretion in mares and to determine the response in two experimental paradigms where gonadotropin secretion is low or elevated. In Experiment 1, conducted during the breeding season (summer), eight long-term ovariectomized mares were treated daily for 21 d with progesterone plus estradiol (n = 4) or oil vehicle. Beginning on Day 14, each mare received, in a randomized design on alternate days, an intravenous injection of either 0, 0.5, 1.0, or 5.0 mg/kg NMA. Treatment with NMA was not accompanied by a change in gonadotropin release in vehicle-treated ovariectomized mares. In steroid-treated mares, gonadotropin values were suppressed compared with vehicle controls, but NMA only induced a significant increase in follicle-stimulating hormone (FSH) and only at the highest dose. In Experiment 2, the effect of the administration of NMA was compared in intact anestrous mares (n = 4) and long-term ovariectomized mares (n = 4) during the nonbreeding season. At the highest doses (1.0 and 5.0 mg/kg), NMA induced a significant increase in luteinizing hormone (LH) and FSH in intact mares. In contrast, pretreatment concentrations of LH and FSH were higher in long-term ovariectomized mares and NMA only increased LH, but not FSH, at the highest dose. The results demonstrate that the administration of NMA stimulates the release of LH and FSH in mares. During anestrus, ovarian intact mares appear more responsive than long-term ovariectomized mares. At high doses, NMA may induce differential release of the gonadotropins in long-term ovariectomized mares.

Effects of N-methyl-D-aspartate (NMDA) on seasonal cycles of reproduction, body weight and pelage colour in the male Siberian hamster

Siberian hamsters (Phodopus sungorus) transferred from stimulatory photoperiods (long days: LD) to inhibitory photoperiods (short days: SD) undergo testicular regression within 8 weeks. This reproductive response to photoperiod was blocked by systemic daily treatment with the glutamatergic agonist N-methyl-D-aspartate (NMDA: 20 mg/kg BW, sc). This powerful effect of NMDA demonstrates the potential for endogenous glutamate to regulate reproductive function. The overall aim of the subsequent studies was to investigate the site and mechanism of action of this glutamatergic agonist in order to identify potential mechanisms through which endogenous glutamate might act. To investigate whether the effect of systemic NMDA was via an effect on the circadian timing system, alterations in gonadal regression and recrudescence, seasonal coat changes (pelage) and body weight (BW) were examined. It would be predicted that long-term cycles of all these seasonal parameters would be affected if the action of NMDA were to perturb the transduction of photoperiodic information. Daily treatments with NMDA, which initially maintained reproductive function in hamsters exposed to SD, did not influence the time course of subsequent testicular recrudescence, nor did they influence long-term cycles of pelage and BW. Moreover, treatment with NMDA induced a dose-dependent increase in serum concentrations of LH within 15 min of systemic injection. These data are consistent with the hypothesis that systemic NMDA exerts it reproductive effects not via an action on the circadian system, but via an action on secretion of GnRH. To investigate potential central sites of action of glutamate, induction of the immediate early gene c-fos, an acute marker of cellular response, was evaluated immunocytochemically (ICC) in brain areas after treatment with NMDA. Although dual-label ICC studies revealed that NMDA did not induce c-fos within GnRH neurons, NMDA did induce c-fos in many cells in the region of the organum vasculosum of the lamina terminalis (OVLT), an area containing a large number of GnRH perikarya, and in the arcuate nucleus, a region close to GnRH secretory terminals in the median eminence. The lack of c-fos induction of GnRH cells argues against a direct effect of NMDA on GnRH neurons. Thus, we examined immunocytochemically the distribution of the common NMDAR1 glutamate receptor subunit to evaluate further the potential sites of glutamatergic action. As expected, NMDAR1-ir was widespread in perikarya throughout the brain, including the region of the OVLT and the arcuate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

A dual-immunocytochemical method to localize c-fos protein in specific neurons based on their content of neuropeptides and connectivity

Enhanced expression of the immediate early gene c-fos has been used as a marker of cellular activation in many different neuronal pathways. We wished to determine the neurochemical content and the connectivity of neurons, in which expression of c-fos is induced. For this purpose, a dual-immunocytochemical staining technique has been developed with avidin-biotin-peroxidase labelling using diaminobenzidine as the chromogen for c-fos protein located in the nucleus, and benzidine dihydrochloride (BDHC) in the presence of sodium nitroprusside to reveal cytoplasmic antigens (neuropeptide or retrograde tracer) in the same section. The blue granular BDHC reaction product in the cytoplasm combined with the homogeneous brown nuclear DAB staining for c-fos protein provides excellent resolution of dual-labelled cells even in tissue sections of 40 microns in thickness. The high sensitivity of the avidin-biotin-peroxidase immunocytochemistry and the stability of the reaction products provide an excellent tool for quantitative analysis of stimulated cells within a neurochemically defined cell group. The BDHC/DAB protocol was developed to identify activated cells in three experimental situations. Firstly, to investigate the phenotype of light-activated cells in the suprachiasmatic nucleus of the hypothalamus, c-fos protein DAB staining was carried out together with BDHC staining for peptide histidine isoleucine (PHI) and vasoactive intestinal peptide (VIP).(ABSTRACT TRUNCATED AT 250 WORDS)