Gonadotropin-inhibitory hormone (GnIH), GnIH receptor and cell signaling

Gonadotropin-inhibitory hormone (GnIH) is an inhibitor of gonadotropin synthesis and release, which was originally identified in the hypothalamus of the Japanese quail (Coturnix japonica). The GnIH precursor polypeptide encodes one GnIH and two GnIH related peptides (GnIH-RP-1 and GnIH-RP-2) in birds that share the same C-terminal LPXRFamide (X=L or Q) motif. The receptor for GnIH is thought to be the G protein-coupled receptor 147 (GPR147) which has been shown to couple predominantly through the Gαi protein to inhibit cAMP production. The crude membrane fraction of COS-7 cells transfected with GPR147 cDNA specifically bound GnIH and GnIH-RPs in a concentration-dependent manner. Scatchard plot analysis of the binding showed that GPR147 possessed a single class of high-affinity binding sites. GnIH neurons project to the median eminence to control anterior pituitary function and GPR147 is expressed in the gonadotropes. GnIH neurons also project to gonadotropin-releasing hormone (GnRH)-I and GnRH-II neurons, and GnRH-I and GnRH-II neurons express GPR147. Thus, GnIH may inhibit gonadotropin synthesis and release by decreasing the activity of GnRH-I neurons as well as directly inhibiting the effects of GnRH on gonadotropes. GnIH may also partially inhibit reproductive behaviors by inhibiting GnRH-II neurons. GnIH and GPR147 are also expressed in the gonads, possibly acting in an autocrine/paracrine manner. The cell signaling process of GPR147 was extensively studied using LβT2 cells, a mouse gonadotrope cell line. In this cell line, mouse GnIH inhibits GnRH-induced gonadotropin subunit, LHβ, FSHβ, and common α, gene transcriptions by inhibiting adenylate cyclase/cAMP/PKA dependent ERK pathway. This review summarizes the functions of GnIH, GnIH receptor and its cell signaling processes in birds and discusses related findings in mammals.

Expression of gonadotropin-releasing hormone II (GnRH-II) receptor in human endometrial and ovarian cancer cells and effects of GnRH-II on tumor cell proliferation

Recently it was shown that a second GnRH system exists in primates. This study was conducted to investigate whether or not the receptor specific for GnRH type II is expressed in human endometrial and ovarian cancer cells and whether or not GnRH-II has effects on tumor cell proliferation. Expression of GnRH-II receptor mRNA in endometrial and ovarian cancer cell lines was demonstrated using RT-PCR and Southern blot analysis. The proliferation of these cell lines was dose- and time-dependently reduced by native GnRH-II. These effects were significantly more potent than the anitproliferative effects of equimolar doses of GnRH-I agonist Triptorelin (p<0.001). In the GnRH-II receptor positive but GnRH-I receptor negative ovarian cancer cell line SK-OV-3 native GnRH-II but not GnRH-I agonist Triptorelin had antiproliferative effects.

LH responses to single doses of exogenous GnRH by freshly captured Damaraland mole-rats, Cryptomys damarensis

Pituitary function in reproductive and nonreproductive colony members of Damaraland mole-rats, Cryptomys damarensis, was investigated by measuring the LH responses to single doses of 2 micrograms exogenous GnRH and physiological saline in 29 females and 37 males (31 of these animals from two entire colonies). In females, basal LH concentrations were significantly greater in reproductive (n = 9) than in nonreproductive animals (n = 11): 7.6 +/- 1.0 versus 4.3 +/- 0.6 miu ml-1, respectively (P < 0.001). Reproductive females had a significantly greater LH response to 2.0 micrograms GnRH (7.6 +/- 1.0 to 37.7 +/- 6.2 miu ml-1; n = 9) than did nonreproductive females (4.3 +/- 0.6 to 11.8 +/- 1.0 miu ml-1; n = 11, P < 0.001). In contrast, there was no significant difference in basal LH concentrations between reproductive (n = 8) and nonreproductive males (n = 20): 5.3 +/- 4.3 versus 3.2 +/- 1.2 miu ml-1, respectively. There was also no difference in LH response to the administration of 2.0 micrograms GnRH between reproductive and nonreproductive males (5.3 +/- 4.3 to 21.8 +/- 8.6 miu ml-1; n = 8; versus 3.2 +/- 1.2 to 21.1 +/- 8.5 miu ml-1; n = 21; P = 0.5). When the results from the two entire colonies were analysed separately, LH responses to GnRH in the 11 nonreproductive females were less than in the two reproductive females. In contrast, the response of two reproductive males in the colonies did not differ from that of 16 nonreproductive males, although these latter comparisons could not be validated statistically.(ABSTRACT TRUNCATED AT 250 WORDS)

Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exist in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfrog. Three different cDNAs, encoding distinct types of bullfrog GnRHR (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximately 30% to approximately 35% with mammalian GnRHRs and approximately 40% to approximately 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has an 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid identity was observed among the three bfGnRHRs. All isolated cDNAs encode functional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, all three receptors exhibited a differential ligand selectivity. For all receptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In conclusion, we demonstrated the presence of three GnRHRs in the bullfrog. Their expression in pituitary and brain suggests that bfGnRHRs play an important role in the regulation of reproductive functions in the bullfrog.

Asn102 of the gonadotropin-releasing hormone receptor is a critical determinant of potency for agonists containing C-terminal glycinamide

We demonstrate a critical role for Asn102 of the human gonadotropin-releasing hormone (GnRH) receptor in the binding of GnRH. Mutation of Asn102, located at the top of the second transmembrane helix, to Ala resulted in a 225-fold loss of potency for GnRH. Eight GnRH analogs, all containing glycinamide C termini like GnRH, showed similar losses of potency between 95- and 750-fold for the [Ala102]GnRHR, compared with wild-type receptor. In contrast, four GnRH analogs that had ethylamide in place of the C-terminal glycinamide residue, showed much smaller decreases in potency between 2.4- and 11-fold. In comparisons of three agonist pairs, differing only at the C terminus, glycinamide derivatives showed an 11-20-fold greater loss of potency for the mutant receptor than their respective ethylamide derivatives. Thus Asn102 is a critical determinant of potency specifically for ligands with C-terminal glycinamide, while ligands with C-terminal ethylamide are less dependent on Asn102. These findings indicate a role for Asn102 in the docking of the glycinamide C terminus and are consistent with hydrogen bonding of the Asn102 side chain with the C-terminal amide moiety. Taken with previous data, they suggest a region of the GnRH receptor formed by the top of helices 2 and 7 as a binding pocket for the C-terminal part of the ligand.

Current and future applications of GnRH, kisspeptin and neurokinin B analogues

Reproductive hormones affect all stages of life from gamete production, fertilization, fetal development and parturition, neonatal development and puberty through to adulthood and senescence. The reproductive hormone cascade has, therefore, been the target for the development of numerous drugs that modulate its activity at many levels. As the central regulator of the cascade, gonadotropin-releasing hormone (GnRH) agonists and antagonists have found extensive applications in treating a wide range of hormone-dependent diseases, such as precocious puberty, prostate cancer, benign prostatic hyperplasia, endometriosis and uterine fibroids, as well as being an essential component of in vitro fertilization protocols. The neuroendocrine peptides that regulate GnRH neurons, kisspeptin and neurokinin B, have also been identified as therapeutic targets, and novel agonists and antagonists are being developed as modulators of the cascade upstream of GnRH. Here, we review the development and applications of analogues of the major neuroendocrine peptide regulators of the reproductive hormone cascade: GnRH, kisspeptin and neurokinin B.

A transcriptionally active human type II gonadotropin-releasing hormone receptor gene homolog overlaps two genes in the antisense orientation on chromosome 1q.12

GnRH-II peptide hormone exhibits complete sequence conservation across vertebrate species, including man. Type-II GnRH receptor genes have been characterized recently in nonhuman primates, but the human receptor gene homolog contains a frameshift, a premature stop codon (UGA), and a 3' overlap of the RBM8A gene on chromosome 1q.12. A retrotransposed pseudogene, RBM8B, retains partial receptor sequence. In this study, bioinformatics show that the human receptor gene promoter overlaps the peroxisomal protein 11-beta gene promoter and the premature UGA is positionally conserved in chimpanzee. A CGA [arginine (Arg)] occurs in porcine DNA, but UGA is shifted one codon to the 5' direction in bovine DNA, suggesting independent evolution of premature stop codons. In contrast to marmoset tissue RNA, exon- and strand-specific probes are required to distinguish differently spliced human receptor gene transcripts in cell lines (HP75, IMR-32). RBM8B is not transcribed. Sequencing of cDNAs for spliced receptor mRNAs showed no evidence for alteration of the premature UGA by RNA editing, but alternative splicing circumvents the frameshift to encode a two-membrane-domain protein before this UGA. A stem-loop motif resembling a selenocysteine insertion sequence and a potential alternative translation initiation site might enable expression of further proteins involved in interactions within the GnRH system.

Type II gonadotrophin-releasing hormone (GnRH-II) in reproductive biology

Humans may be particularly unusual with respect to the gonadotrophin-releasing hormone (GnRH) control of their reproductive axis in that they possess two distinct GnRH precursor genes, on chromosomes 8p11-p21 and 20p13, but only one conventional GnRH receptor subtype (type I GnRH receptor) encoded within the genome, on chromosome 4. A disrupted human type II GnRH receptor gene homologue is present on chromosome 1q12. The genes encoding GnRH ligand precursors and GnRH receptors have now been characterized in a broad range of vertebrate species, including fish, amphibians and mammals. Ligand precursors and receptors can be categorized into three phylogenetic families. Members of each family exist in primitive vertebrates, whereas mammals exhibit selective loss of ligand precursor and receptor genes. One interpretation of these findings is that each ligand-cognate receptor family may have evolved to fulfil a separate function in reproductive physiology and that species-specific gene inactivation, modification or loss may have occurred during evolution when particular roles have become obsolete or subject to regulation by a different biochemical pathway. Evidence in support of this concept is available following the characterization of the chromosomal loci encoding the human type II GnRH receptor homologue, a rat type II GnRH receptor gene remnant (on rat chromosome 18) and a mouse type II GnRH ligand precursor gene remnant (on mouse chromosome 2). Whether type I GnRH and type II GnRH peptides elicit different signalling responses in humans by activation of the type I GnRH receptor in a cell type-specific fashion remains to be shown. Recent structure-function studies of GnRH ligands and GnRH receptors and their expression patterns in different tissues add further intrigue to this hypothesis by indicating novel roles for GnRH such as neuromodulation of reproductive function and direct regulation of peripheral reproductive tissues. Surprises concerning the complexities of GnRH ligand and receptor function in reproductive endocrinology should continue to emerge in the future.

Immunocytochemical localization of GnRH precursor in the hypothalamus of European starlings during sexual maturation and photorefractoriness

Immunocytochemistry with quantitative image analysis, for both GnRH and its precursor proGnRH-GAP, was used in male European starlings (Sturnus vulgaris) to investigate four stages of a photoperiodically-induced reproductive cycle. Four different groups of birds were examined: photosensitive buy sexually immature, sexually mature, undergoing gonadal regression, and after the completion of regression and fully photorefractory. The size of cells staining for GnRH and proGnRH-GAP increased during gonadal maturation. A reduction in the number of cells staining for GnRH and the size of cells staining for both GnRH and proGnRH-GAP occurred during gonadal regression, though staining for GnRH and proGnRH-GAP in the median eminence remained high at this stage. Birds examined after completion of regression showed significantly reduced staining for both GnRH and its precursor. These observations suggest that photorefractoriness is promoted by a reduction in proGnRH-GAP production and in GnRH synthesis, rather than requiring inhibition of release of GnRH at the median eminence.

Receptor binding and gonadotropin-releasing activity of a novel chicken gonadotropin-releasing hormone ([His5, Trp7, Tyr8]GnRH) and a D-Arg6 analog

Receptor binding and gonadotropin-releasing activity was compared for mammalian GnRH, [Gln8]GnRH (chicken I GnRH), [His5, Trp7, Tyr8]GnRH (chicken II GnRH), [Trp7, Leu8]GnRH (salmon GnRH), and [D-Arg6] chicken II GnRH. The mean ED50 values for mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH in stimulating LH release from dispersed chicken pituitary cells were 0.27 nM, 0.28 nM, 0.055 nM, and 0.11 nM, respectively. The relative potencies of the peptides compared in the same assay were 0.93, 1.0, 5.6, and 2.5. The ED50 values for chicken I GnRH, chicken II GnRH, and salmon GnRH in stimulating FSH release were 0.37 nM, 0.034 nM, and 0.18 nM, and the relative potencies were 1.0, 13.5, and 1.8. Chicken II GnRH was, therefore, more potent than chicken I GnRH and mammalian GnRH in releasing LH and appeared to have an even greater relative FSH-releasing activity than chicken I GnRH or mammalian GnRH. Introduction of D-Arg6 into chicken II GnRH enhanced the activity of this analog 4- and 2-fold relative to chicken II GnRH in LH- and FSH-releasing activity, respectively. The ED50 values of mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH in releasing LH from cultured sheep pituitary cells were 2.9 nM, 96 nM, 22 nM, and 104 nM, respectively. The relative potencies were 1.0, 0.016, 0.084, and 0.047. Introduction of D-Arg6 into chicken II GnRH enhanced activity 9-fold. In a rat pituitary receptor binding assay the ED50 values of mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH were 2.9 nM, 1480 nM, 19 nM, and 258 nM, respectively. [D-Arg6]Chicken II GnRH was 46 times more active than the natural chicken II GnRH peptide. The results show: 1) chicken II GnRH is more potent than chicken I GnRH, which is equipotent with mammalian GnRH in releasing LH from chicken pituitary cells. Chicken II GnRH is even more potent at releasing FSH. 2) Salmon GnRH is also more potent than chicken I GnRH and mammalian GnRH in stimulating gonadotropin release from chicken pituitary cells. It appears, therefore, that Trp in the 7 position contributes to the enhanced activity of salmon and chicken II GnRH. 3) The low activity of chicken I GnRH, chicken II GnRH, and salmon GnRH in the sheep pituitary cell bioassay and rat pituitary receptor binding assay confirms that Arg8 in mammalian GnRH is important for activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothesis: kisspeptin mediates male hypogonadism in obesity and type 2 diabetes

Hypogonadism occurs commonly in men with type 2 diabetes (T2DM) and severe obesity. Current evidence points to a decreased secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus and thereby decreased secretion of gonadotropins from the pituitary gland as a central feature of the pathophysiology in these men. Hyperglycaemia, inflammation, leptin and oestrogen-related feedback have been proposed to make aetiological contributions to the hypogonadotropic hypogonadism of T2DM. However, the neuroendocrine signals that link these factors with modulation of GnRH neurons have yet to be identified. Kisspeptins play a central role in the modulation of GnRH secretion and, thus, downstream regulation of gonadotropins and testosterone secretion in men. Inactivating mutations of the kisspeptin receptor have been shown to cause hypogonadotropic hypogonadism in man, whilst an activating mutation is associated with precocious puberty. Data from studies in experimental animals link kisspeptin expression with individual factors known to regulate GnRH secretion, including hyperglycaemia, inflammation, leptin and oestrogen. We therefore hypothesise that decreased endogenous kisspeptin secretion is the common central pathway that links metabolic and endocrine factors in the pathology of testosterone deficiency seen in men with obesity and T2DM. We propose that the kisspeptin system plays a central role in integrating a range of metabolic inputs, thus constituting the link between energy status with the hypothalamic-pituitary-gonadal axis, and put forward potential clinical studies to test the hypothesis.

Evolution of gonadotropin-releasing hormones

GnRH was originally isolated as a hypothalamic peptide hormone that regulates the reproductive system by stimulating the release of gonadotropins from the anterior pituitary. However, multiple molecular forms of the peptide have evolved, which have been coopted for a variety of regulatory functions: as a neurotransmitter in the central and sympathetic nervous systems, as a paracrine regulator in the gonads and placenta, and as an autocrine regulator in tumor cells. We review here the evolution of these variant forms of GnRH and their functions.

Suppression of the GnRH pulse generator by neurokinin B involves a κ-opioid receptor-dependent mechanism

Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with kisspeptin, Dynorphin A (Dyn), and their receptors [G-protein-coupled receptor-54 (GPR54)] and κ-opioid receptor (KOR), respectively] within kisspeptin/NKB/Dyn (KNDy) neurons in the hypothalamic arcuate nucleus (ARC), the proposed site of the GnRH pulse generator. Much previous research has employed intracerebroventricular (icv) administration of KNDy agonists and antagonists to address the functions of KNDy neurons. We performed a series of in vivo neuropharmacological experiments aiming to determine the role of NKB/NK3R signaling in modulating the GnRH pulse generator and elucidate the interaction between KNDy neuropeptide signaling systems, targeting our interventions to ARC KNDy neurons. First, we investigated the effect of intra-ARC administration of the selective NK3R agonist, senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain blood samples from freely moving ovariectomized 17β-estradiol-replaced rats. Our results show that senktide suppresses LH pulses in a dose-dependent manner. Intra-ARC administration of U50488, a selective KOR agonist, also caused a dose-dependent, albeit more modest, decrease in LH pulse frequency. Thus we tested the hypothesis that Dyn/KOR signaling localized to the ARC mediates the senktide-induced suppression of the LH pulse by profiling pulsatile LH secretion in response to senktide in rats pretreated with nor-binaltorphimine, a selective KOR antagonist. We show that nor-binaltorphimine blocks the senktide-induced suppression of pulsatile LH secretion but does not affect LH pulse frequency per se. In order to address the effects of acute activation of ARC NK3R, we quantified (using quantitative RT-PCR) changes in mRNA levels of KNDy-associated genes in hypothalamic micropunches following intra-ARC administration of senktide. Senktide down-regulated expression of genes encoding GnRH and GPR54 (GNRH1 and Kiss1r, respectively), but did not affect the expression of Kiss1 (which encodes kisspeptin). We conclude that NKB suppresses the GnRH pulse generator in a KOR-dependent fashion and regulates gene expression in GnRH neurons.

Absence of rapid desensitization of the mouse gonadotropin-releasing hormone receptor

Desensitization of gonadotropin release by the pituitary gland in response to gonadotropin-releasing hormone (GnRH) agonists has clinical applications in the treatment of gonadal-hormone-dependent disorders. We therefore investigated possible desensitization of inositol phosphate (IP) responses of GNRH receptors. No short-term homologous desensitization of the IP response to GnRH was observed in either alpha T3 gonadotrope cells line or GH3 cells transfected with GnRH receptor cDNA. The absence of homologous desensitization is unusual among G-protein-coupled receptors, and may be due to the absence of a C-terminal cytoplasmic tail, a unique feature of the GnRH receptor. Several potential protein kinase C phosphorylation sites which might mediate heterologous desensitization are present on the GnRH receptor. In both alpha T3 cells and GnRH-receptor-transfected Cos-1 cells, activation of protein kinase C by pretreatment with phorbol ester caused a 35-53% decrease in the IP response to GnRH. However, phorbol ester also inhibited guanosine 5'-[gamma-thio]triphosphate-stimulated IP production in permeabilized Cos-1 cells, suggesting that this inhibition is mediated at a post-receptor site.

Identification of N-glycosylation sites in the gonadotropin-releasing hormone receptor: role in receptor expression but not ligand binding

The asparagine residues of the three N-glycosylation consensus sequences in the mouse gonadotropin-releasing hormone receptor were mutated to determine which residues were glycosylated and the function of glycosylation. Photoaffinity labelled Gln4 and Gln18 receptor mutants exhibited lower apparent molecular weight on SDS polyacrylamide gel electrophoresis, while the Gln102 receptor showed wildtype mobility. This indicates that the receptor is glycosylated at Asn4 and Asn18 but not at Asn102. Binding affinities of all the mutant receptors were normal, indicating that carbohydrate moieties are not involved in ligand binding interactions. However, expression of the Gln4 and Gln18 receptors were substantially decreased, indicating a role for glycosylation in receptor expression or stability. All the glycosylation site mutants were capable of normal signal transduction, as indicated by their ability to stimulate inositol phosphate production.

Defective migration of neuroendocrine GnRH cells in human arrhinencephalic conditions

Patients with Kallmann syndrome (KS) have hypogonadotropic hypogonadism caused by a deficiency of gonadotropin-releasing hormone (GnRH) and a defective sense of smell related to olfactory bulb aplasia. Based on the findings in a fetus affected by the X chromosome–linked form of the disease, it has been suggested that hypogonadism in KS results from the failed embryonic migration of neuroendocrine GnRH1 cells from the nasal epithelium to the forebrain. We asked whether this singular observation might extend to other developmental disorders that also include arrhinencephaly. We therefore studied the location of GnRH1 cells in fetuses affected by different arrhinencephalic disorders, specifically X-linked KS, CHARGE syndrome, trisomy 13, and trisomy 18, using immunohistochemistry. Few or no neuroendocrine GnRH1 cells were detected in the preoptic and hypothalamic regions of all arrhinencephalic fetuses, whereas large numbers of these cells were present in control fetuses. In all arrhinencephalic fetuses, many GnRH1 cells were present in the frontonasal region, the first part of their migratory path, as were interrupted olfactory nerve fibers that formed bilateral neuromas. Our findings define a pathological sequence whereby a lack of migration of neuroendocrine GnRH cells stems from the primary embryonic failure of peripheral olfactory structures. This can occur either alone, as in isolated KS, or as part of a pleiotropic disease, such as CHARGE syndrome, trisomy 13, and trisomy 18.

Potential roles of the prokineticins in reproduction

Prokineticins are multifunctional secreted proteins that were originally identified as regulators of intestinal contraction but subsequently shown to affect vascular function, hyperalgesia, spermatogenesis, neuronal survival, circadian rhythm, nociception, feeding behaviour, immune responses, haematopoiesis and the development of the olfactory and gonadotropin-releasing hormone systems. Their role in the reproductive tract is still not fully elucidated, although they are reputed to increase microvascular permeability. Expression of prokineticins and their receptors has been reported in the ovary, uterus, placenta, testis and prostate. Their expression has also been reported in various pathologies of the reproductive tract, and future studies will highlight whether inhibition of prokineticin function in these pathologies would be a useful therapeutic target.

Amygdala Kisspeptin Neurons: Putative Mediators of Olfactory Control of the Gonadotropic Axis

Kisspeptins and their receptors are potent regulators of the gonadotropic axis. Kisspeptin neurons are found mainly in the hypothalamic arcuate nucleus and the anteroventral periventricular nucleus. However, there is also a third population of kisspeptin neurons, located in the amygdala. We used fluorescence immunohistochemistry to quantify and localize the amygdala kisspeptin neurons and to reveal close apposition and putative innervations by vasopressinergic and tyrosine hydroxylase-positive dopaminergic neurons. Using microinjections of retro- and anterograde tracers, and viral transfection systems in rats and transgenic mice, we showed reciprocal connectivity between the accessory olfactory bulb and the amygdala kisspeptin neurons. In vitro recordings indicate an inhibitory action of kisspeptin on mitral cells in the accessory olfactory bulb. Using viral specific-cell gene expression in transgenic mice in combination with double immunofluorescence histochemistry, we found that the amygdala kisspeptin neurons also project to gonadotropin-releasing hormone (GnRH) neurons in the preoptic area. Our neuroanatomical and electrophysiological data suggest that amygdala kisspeptin neurons integrate social behaviour and odour information into GnRH neurons in the preoptic area to coordinate the gonadotropic axis and the appropriate output behaviour to odour cues.

The functional microdomain in transmembrane helices 2 and 7 regulates expression, activation, and coupling pathways of the gonadotropin-releasing hormone receptor

Structural microdomains of G protein-coupled receptors (GPCRs) consist of spatially related side chains that mediate discrete functions. The conserved helix 2/helix 7 microdomain was identified because the gonadotropin-releasing hormone (GnRH) receptor appears to have interchanged the Asp(2.50) and Asn(7.49) residues which are conserved in transmembrane helices 2 and 7 of rhodopsin-like GPCRs. We now demonstrate that different side chains of this microdomain contribute specifically to receptor expression, heterotrimeric G protein-, and small G protein-mediated signaling. An Asn residue is required in position 2.50(87) for expression of the GnRH receptor at the cell surface, most likely through an interaction with the conserved Asn(1.50(53)) residue, which we also find is required for receptor expression. Most GPCRs require an Asp side chain at either the helix 2 or helix 7 locus of the microdomain for coupling to heterotrimeric G proteins, but the GnRH receptor has transferred the requirement for an acidic residue from helix 2 to 7. However, the presence of Asp at the helix 7 locus precludes small G protein-dependent coupling to phospholipase D. These results implicate specific components of the helix 2/helix 7 microdomain in receptor expression and in determining the ability of the receptor to adopt distinct activated conformations that are optimal for interaction with heterotrimeric and small G proteins.

Mast cells with gonadotropin-releasing hormone-like immunoreactivity in the brain of doves

Using an antiserum (LR-1) raised against mammalian gonadotropin-releasing hormone (GnRH), we previously identified a nonneuronal cell that was more numerous in the medial habenula (MH) of courting ring doves than in individuals housed in visual isolation. The current studies suggest that they are mast cells. Both acidic toluidine blue and toluidine blue dissolved in water/butanediol revealed metachromatic cells with a distribution and morphology similar to that obtained by immunostaining with the GnRH antiserum in the MH. Some cells had granules reactive to safranin in the presence of alcian blue, indicative of a highly sulfated proteoglycan of the heparin family. Immunocytochemical studies demonstrated that all MH cells containing GnRH-like immunoreactivity contained histamine, another mast cell marker. The GnRH-immunoreactive cells had a unilobular, ovoid nucleus. Secretory granules within the cells were electron dense and displayed a variety of internal structures. Fine filamentous processes appeared evenly distributed on the cell surface whether cells were located on the pial surface or within the brain parenchyma. All of these features are characteristic of mast cells. To test whether the epitope recognized by the GnRH antiserum was produced by the mast cells or endocytosed from the cerebrospinal fluid, an iodinated GnRH analog was injected intracerebroventricularly at the initiation of courtship. Radioautography revealed no radioactive cells in the brain, indicating that the GnRH antibody recognized a molecule synthesized by the nonneuronal cells rather than internalized by a receptor-mediated mechanism. These observations suggest an interaction between a component of the immune network and specific regions of the central nervous system.

Gonadotropin-releasing hormone II stimulates female sexual behavior in marmoset monkeys

GnRH II (pGlu-His-Trp-Ser-Try-Gly-Leu-Arg-Pro-GlyNH2), an evolutionarily conserved member of the GnRH family, stimulates reproductive behavior in a number of vertebrates. To explore a role for GnRH II in regulating primate sexual behavior, eight adult female common marmosets, each fitted with an indwelling intracerebroventricular (icv) cannula, were ovariectomized, implanted subcutaneously with empty (n = 4) or estradiol-filled (n = 4) SILASTIC brand capsules, and pair housed with an adult male mate. After icv infusion of vehicle or peptides, females were placed in an observation cage for 90 min, out of visual contact with other marmosets, before the 30-min behavioral test with their male partner. Compared with vehicle, GnRH II (1 and 10 microg) increased the total number of proceptive (sexual solicitation) behaviors (tongue flicking, proceptive stares, and frozen postures) exhibited by females toward their pair mates and specifically increased the frequency of freeze postures. Effects were maximal at 1 microg and not dependent upon estradiol supplementation. GnRH II agonists/GnRH I antagonists 135-18 (1 microg) and 132-25 (1 microg), which stimulate inositol phosphate production via the marmoset type II receptor, increased the frequency of total proceptive behavior but did not specifically stimulate freeze-posture behavior. In contrast, GnRH I, at 1 mug, did not alter the frequency of proceptive behaviors. Female receptivity (female compliance with male sexual behavior) was not altered by any of the peptides tested. These findings implicate a role for GnRH II and the cognate GnRH type II receptor in stimulating female marmoset sexual behavior.

Contrasting internalization kinetics of human and chicken gonadotropin-releasing hormone receptors mediated by C-terminal tail

The chicken gonadotropin-releasing hormone receptor (GnRH-R) is notable for having a cytoplasmic C-terminal tail, which is not present in the mammalian GnRH-Rs. We report here that the cytoplasmic tail mediates rapid agonist-promoted receptor internalization. The chicken GnRH-R mediated internalization of gonadotropin-releasing hormone (GnRH) agonist (125I[His5-D-Tyr6]GnRH) at a rate of 11.3%.min-1, compared with only 0.71 %.min-1 for the human GnRH-R. To determine whether the presence of the cytoplasmic tail was responsible for the more rapid internalization kinetics of the chicken GnRH-R we truncated the tail after the Ile336 residue (S337stop). Receptor-mediated internalization of GnRH agonist by the S337stop-chicken GnRH-R was much slower than in the wild-type chicken receptor, and was similar to the wild-type human GnRH-R (0.55 %.min-1). These data indicate that rapid agonist-promoted internalization of the chicken GnRH-R is mediated through elements in the cytoplasmic C-terminal tail, distal to or including Ser337 and suggests that elimination of the C-terminal tail during evolution of mammalian GnRH-Rs may be related to its effects on internalization.

Estradiol stimulates preoptic area-anterior hypothalamic proGnRH-GAP gene expression in ovariectomized rats

The decapeptide gonadotropin-releasing hormone (GnRH) and the 56-amino acid GnRH-associated peptide (GAP) are derived from a common precursor translated from the proGnRH-GAP mRNA. Studies using solid-phase hybridization techniques (i.e., Northern blot analysis, dot blot analysis, or in situ hybridization autoradiography) have yielded a controversy as to whether estradiol stimulates, inhibits, or has any effect on proGnRH-GAP gene expression in the preoptic area-anterior hypothalamus (POA-AH) of the ovariectomized (OVX) rat. Using a sensitive and quantitative solution hybridization-nuclease protection assay, which ensures complete hybridization of target RNA to probe RNA, we examined the effects of OVX and estradiol replacement on the amount of proGnRH-GAP mRNA in individual POA-AH dissections. Rats sacrificed at different intervals after OVX showed a significant time-dependent decrease (34-60%) in the levels of POA-AH proGnRH-GAP mRNA relative to sham-operated animals; OVX rats treated with estradiol, however, had proGnRH-GAP mRNA levels comparable to those of sham-OVX animals. To verify these observations, levels of the proGnRH-GAP peptide, measured by radioimmunoassay with antibodies directed against the cleavage and amidation site between the GnRH and the GAP portions fo the precursor molecular, were also found to decrease (37%) after OVX and increase (63-85%) following estradiol replacement, relative to intact rats. These data support the view that estradiol stimulates the levels of both proGnRH-GAP mRNA and its primary translation product in the POA-AH region of the OVX rat.

Comparative sequence analysis and functional characterization of the cloned sheep gonadotropin-releasing hormone receptor reveal differences in primary structure and ligand specificity among mammalian receptors

The cloned sheep gonadotropin-releasing hormone (GnRH) receptor was analysed for sequence homology/differences among mammalian receptors and its pharmacology characterized in COS-1 cells. Transmembrane domains TM2, TM3, TM5, TM6 and TM7, and extracellular loop 1 are most highly conserved (> 90%) in this G-protein coupled receptor. The Kd of the sheep receptor in binding assays (4.9 nM) was similar to the human and rat receptors, but lower than the mouse receptor. The rank order of potency of a series of GnRH analogues for binding and inositol phosphate stimulation in transfected COS-1 cells was identical to that of the receptor characterized in sheep pituitary gonadotropes. Northern blot analysis identified four transcripts sized 5.4 kb, 3.6 kb, 2.3 kb and 1.3 kb in sheep pituitaries which were upregulated by castration.