Neuropeptide Y influences acute food intake and energy status affects NPY immunoreactivity in the female musk shrew (Suncus murinus)

Neuropeptide Y (NPY) stimulates feeding, depresses sexual behavior, and its expression in the brain is modulated by energetic status. We examined the role of NPY in female musk shrews, a species with high energetic and reproductive demands; they store little fat, and small changes in energy can rapidly diminish or enhance sexual receptivity. Intracerebroventricular infusion of NPY enhanced acute food intake in shrews; however, NPY had little affect on sexual receptivity. The distribution of NPY immunoreactivity in the female musk shrew brain was unremarkable, but energy status differentially affected NPY immunoreactivity in several regions. Similar to what has been noted in other species, NPY immunoreactivity was less dense in brains of ad libitum shrews and greater in shrews subjected to food restriction. In two midbrain regions, both of which contain high levels of gonadotropin releasing hormone II (GnRH II), which has anorexigenic actions in shrews, NPY immunoreactivity was more sensitive to changes in food intake. In these regions, acute re-feeding (90-180 min) after food restriction reduced NPY immunoreactivity to levels noted in ad libitum shrews. We hypothesize that interactions between NPY and GnRH II maintain energy homeostasis and reproduction in the musk shrew.     

Inhibitory effect of chicken gonadotropin-releasing hormone II on food intake in the goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is an evolutionarily conserved neuropeptide with 10 amino acid residues, which possesses some structural variants. A molecular form known as chicken GnRH II ([His⁵ Trp⁷ Tyr⁸] GnRH, cGnRH II) is widely distributed in vertebrates, and has recently been implicated in the regulation of sexual behavior and food intake in an insectivore, the musk shrew. However, the influence of cGnRH II on feeding behavior has not yet been studied in model animals such as rodents and teleost fish. In this study, therefore, we investigated the role of cGnRH II in the regulation of feeding behavior in the goldfish, and examined its involvement in food intake after intracerebroventricular (ICV) administration. ICV-injected cGnRH II at graded doses, from 0.1 to 10 pmol/g body weight (BW), induced a decrease of food consumption in a dose-dependent manner during 60 min after treatment. Cumulative food intake was significantly decreased by ICV injection of cGnRH II at doses of 1 and 10 pmol/g BW during the 60-min post-treatment observation period. ICV injection of salmon GnRH ([Trp⁷ Leu⁸] GnRH, sGnRH) at doses of 0.1-10 pmol/g BW did not affect food intake. The anorexigenic action of cGnRH II was completely blocked by treatment with the GnRH type I receptor antagonist, Antide. However, the anorexigenic action of cGnRH II was not inhibited by treatment with the corticotropin-releasing hormone (CRH) 1/2 receptor antagonist, α-helical CRH₍₉₋₄₁₎, and the melanocortin 4 receptor antagonist, HS024. These results suggest that, in the goldfish, cGnRH II, but not sGnRH, acts as an anorexigenic factor, as is the case in the musk shrew, and that the anorexigenic action of cGnRH II is independent of CRH- and melanocortin-signaling pathways.     

Effects of gonadotropin-releasing hormones, corticotropin-releasing hormone, and vasopressin on female sexual behavior

The effects of intracerebroventricular (icv) infusion of four neuropeptides on female sexual behavior were examined in the female musk shrew (Suncus murinus). In the first experiment, (icv) infusion of 100 ng of the mammalian form of gonadotropin-releasing hormone (mGnRH) facilitated rapid display of receptivity. Gonadotropin-releasing hormone-infused females had shorter latencies to rump present and tail wag, compared with controls. In a second experiment, icv administration of the other form of GnRH present in musk shrew brain, the chicken GnRH-II form, produced no changes in female behavior relative to the control condition. In Experiment 3, icv delivery of corticotropin-releasing hormone (CRH) facilitated female sexual behavior, relative to vasopressin and controls. The females treated with CRH had shorter latencies to display rump present, tail wag, and for the receipt of the first missed intromission compared with females in the other treatment groups. Vasopressin increased female scent marking relative to that of CRH-treated females. These data indicate that neurohormones of the hypothalamic-pituitary-gonadal and the hypothalamic-pituitary-adrenal axes can facilitate reproductive behavior in S. murinus.     

Brief refeeding restores reproductive readiness in food-restricted female musk shrews (Suncus murinus)

Food deprivation blocks sexual behavior and disrupts estrous cycles in mammals. We asked whether reduced copulatory behavior, produced by limited food intake, could be reversed by brief refeeding intervals in the female musk shrew. In Experiment 1, animals were food restricted to 60% of ad lib (FR), and an additional group of FR females were refed for 90 min prior to testing (RF). Refed and ad lib (AL) fed females were significantly more likely to mate than FR females. To test the hypothesis that food-induced restoration of copulatory behavior is not the result of changes in peripheral steroids, we repeated Experiment 1 using ovariectomized and testosterone-implanted females. The results from Experiment 2 were similar to those found in the first study. Next, a more severe refeeding schedule was employed; females were restricted to 50% of ad lib intake. Females in the RF and FR groups were significantly less likely than the AL animals to mate. In the last experiment, females were food restricted to 50% and longer refeeding intervals were employed. Four and one-half hours of food intake did not reinstate sexual behavior, but females refed for 12 h were as likely to mate as ad lib fed controls. We also did not detect any differences in plasma concentrations of testosterone and cortisol in AL, FR, and RF ovary-intact animals. These results define a nutritional threshold for copulatory behavior in the musk shrew. Since this species is highly sensitive to small alterations in food intake, it is a useful model for studies of interactions between metabolic fuels and behavior.     

Acute re-feeding reverses food restriction-induced hypothalamic-pituitary-gonadal axis deficits

Undernutrition has well-established effects on female reproduction. Here we describe the effects of food restriction on aspects of the hypothalamic-pituitary-gonadal (HPG) axis in the female musk shrew. We determined that acute re-feeding reverses deficits brought on by food restriction. Two days of food restriction led to an increase in proGnRH immunoreactive cells in the preoptic area relative to ad libitum-fed controls (AL). This increase was reversed by 90 min of ad libitum feeding in the re-fed females (RF). In addition, food-restricted (FR) females had significantly greater GnRH content in the median eminence than either the AL or RF females. After GnRH was administered, the majority of females in all food conditions ovulated, yet the FR females had significantly fewer corpora lutea than either the AL or RF animals. These data show that food restriction impairs HPG axis function in female musk shrews, and that some of these impairments can be rapidly reversed by acute re-feeding.     

FoxO1 target Gpr17 activates AgRP neurons to regulate food intake

Hypothalamic neurons expressing Agouti-related peptide (AgRP) are critical for initiating food intake, but druggable biochemical pathways that control this response remain elusive. Thus, genetic ablation of insulin or leptin signaling in AgRP neurons is predicted to reduce satiety but fails to do so. FoxO1 is a shared mediator of both pathways, and its inhibition is required to induce satiety. Accordingly, FoxO1 ablation in AgRP neurons of mice results in reduced food intake, leanness, improved glucose homeostasis, and increased sensitivity to insulin and leptin. Expression profiling of flow-sorted FoxO1-deficient AgRP neurons identifies G-protein-coupled receptor Gpr17 as a FoxO1 target whose expression is regulated by nutritional status. Intracerebroventricular injection of Gpr17 agonists induces food intake, whereas Gpr17 antagonist cangrelor curtails it. These effects are absent in Agrp-Foxo1 knockouts, suggesting that pharmacological modulation of this pathway has therapeutic potential to treat obesity.     

Gonadotropin-releasing hormone II (GnRH II) mediates the anorexigenic actions of α-melanocyte-stimulating hormone (α-MSH) and corticotropin-releasing hormone (CRH) in goldfish

Intracerebroventricular (ICV) administration of gonadotropin-releasing hormone II (GnRH II), which plays a crucial role in the regulation of reproduction in vertebrates, markedly reduces food intake in goldfish. However, the neurochemical pathways involved in the anorexigenic action of GnRH II and its interaction with other neuropeptides have not yet been identified. Alpha-melanocyte-stimulating hormone (α-MSH), corticotropin-releasing hormone (CRH) and CRH-related peptides play a major role in feeding control as potent anorexigenic neuropeptides in goldfish. However, our previous study has indicated that the GnRH II-induced anorexigenic action is not blocked by treatment with melanocortin 4 receptor (MC4R) and CRH receptor antagonists. Therefore, in the present study, we further examined whether the anorexigenic effects of α-MSH and CRH in goldfish could be mediated through the GnRH receptor neuronal pathway. ICV injection of the MC4R agonist, melanotan II (80 pmol/g body weight; BW), significantly reduced food intake, and its anorexigenic effect was suppressed by ICV pre-administration of the GnRH type I receptor antagonist, antide (100 pmol/g BW). The CRH-induced (50 pmol/g BW) anorexigenic action was also blocked by treatment with antide. ICV injection of CRH (50 pmol/g BW) induced a significant increase of the GnRH II mRNA level in the hypothalamus, while ICV injection of melanotan II (80 pmol/g BW) had no effect on the level of GnRH II mRNA. These results indicate that, in goldfish, the anorexigenic actions of α-MSH and CRH are mediated through the GnRH type I receptor-signaling pathway, and that the GnRH II system regulates feeding behavior.     

Feeding response to ghrelin agonist and antagonist in lean and obese Zucker rats

Ghrelin is a new orexigenic and adipogenic peptide primarily produced by the stomach and the hypothalamus. In the present experiment, we determined the circulating ghrelin levels in 60-week old fa/fa Zucker rats with a well-established obesity (n = 12) and in their lean (FA/FA) counterparts (n = 12). We also tested the feeding response of both groups to intra-peritoneal (I.P.) injection of ghrelin agonist and antagonist. Obese rats ate significantly more than the lean rats (21.7 +/- 1.1 vs. 18.3 +/- 0.3 g/day; p < 0.01). Their plasma ghrelin concentration was 35% higher than that in the lean homozygous rats (p < 0.025). GHRP-6 (1 mg/kg I.P, a GHS-R agonist) stimulated food intake in lean but not in obese rats (p < 0.01), whereas [D-Lys)]-GHRP-6 (12 mg/kg I.P., a GHS-R antagonist) decreased food intake in both groups (p < 0.0001). These results indicate that the obese Zucker rat is characterized by an increase in plasma ghrelin concentrations and by an attenuated response to a GHS-R agonist. They support a role for ghrelin in the development of obesity in the absence of leptin signaling.     

Effects of galanin-like peptide (GALP) on locomotion, reproduction, and body weight in female and male mice

Galanin-like peptide (GALP) has been implicated in the neuroendocrine regulation of both feeding and reproduction. In male rodents and primates, intracerebroventricular (icv) infusions of GALP stimulate luteinizing hormone (LH) release, induce Fos expression in brain areas implicated in feeding and reproduction, and affect food intake and body weight in rodents. In gonad-intact and castrated male rats, icv administration of GALP also stimulates male sexual behavior. While the effects of GALP on male physiology and behavior are well documented, no studies have addressed such a role of GALP in females. We tested the effects of icv GALP infusions on LH release, locomotor activity, motor control, and body weight regulation in adult ovariectomized female mice hormonally primed with estradiol benzoate and progesterone. In addition, sexually-experienced male and female mice were treated with GALP and tested for sexual behavior. In females, GALP reduced open-field locomotor activity, the ability to maintain grip on an accelerating rotarod, and 24-h body weight in a dose-dependent manner. GALP also increased LH secretion in female mice, an effect that was blocked by pre-treatment with Antide, a gonadotropin-releasing hormone (GnRH) type-1 receptor antagonist. GALP infusions slightly decreased the occurrence of lordosis behavior in female mice and significantly increased the latencies with which females displayed receptivity. Unlike previous reports in male rats, GALP inhibited male sexual behavior in mice. Our data indicate that in female mice, GALP stimulates LH release via GnRH, and decreases body weight, motor control, and locomotor activity via GnRH-independent pathways. Furthermore, our sexual behavior and locomotor findings suggest species-specific differences in the mechanism and/or location of GALP action in the brains of rats and mice.     

Gonadotropin-releasing hormone analog structural determinants of selectivity for inhibition of cell growth: support for the concept of ligand-induced selective signaling

GnRH and its receptor are expressed in human reproductive tract cancers, and direct antiproliferative effects of GnRH analogs have been demonstrated in cancer cell lines. The intracellular signaling responsible for this effect differs from that mediating pituitary gonadotropin secretion. The GnRH structure-activity relationship is different for the two effects. Here we report a structure-activity relationship study of GnRH agonist antiproliferative action in model cell systems of rat and human GnRH receptors stably expressed in HEK293 cells. GnRH II was more potent than GnRH I in inhibiting cell growth in the cell lines. In contrast, GnRH I was more potent than GnRH II in stimulating inositol phosphate production, the signaling pathway in gonadotropes. The different residues in GnRH II (His(5), Trp(7), Tyr(8)) were introduced singly or in pairs into GnRH I. Tyr(5) replacement by His(5) produced the highest increase in the antiproliferative potency of GnRH I. Tyr(8) substitution of Arg(8) produced the most selective analog, with very poor inositol phosphate generation but high antiproliferative potency. In nude mice bearing tumors of the HEK293 cell line, GnRH II and an antagonist administration was ineffective in inhibiting tumor growth, but D-amino acid stabilized analogs (D-Lys(6) and D-Arg(6)) ablated tumor growth. Docking of GnRH I and GnRH II to the human GnRH receptor molecular model revealed that Arg(8) of GnRH I makes contact with Asp(302), whereas Tyr(8) of GnRH II appears to make different contacts, suggesting these residues stabilize different receptor conformations mediating differential intracellular signaling and effects on gonadotropin and cell growth. These findings provide the basis for the development of selective GnRH analog cancer therapeutics that directly target tumor cells or inhibit pituitary gonadotropins or do both.     

Microevolution of neuroendocrine mechanisms regulating reproductive timing in Peromyscus leucopus

A key question in the evolution of life history and in evolutionary physiology asks how reproductive and other life-history traits evolve. Genetic variation in reproductive control systems may exist in many elements of the complex inputs that can affect the hypothalamic-pituitary-gonadal (HPG) or reproductive axis. Such variation could include numbers and other traits of secretory cells, the amount and pattern of chemical message released, transport and clearance mechanisms, and the number and other traits of receptor cells. Selection lines created from a natural population of white-footed mice (Peromyscus leucopus) that contains substantial genetic variation in reproductive inhibition in response to short winter daylength (SD) have been used to examine neuroendocrine variation in reproductive timing. We hypothesized that natural genetic variation would be most likely to occur in the inputs to GnRH neurons and/or in GnRH neurons themselves, but not in elements of the photoperiodic pathway that would have pleiotropic effects on nonreproductive functions as well as on reproductive functions. Significant genetic variation has been found in the GnRH neuronal system. The number of GnRH neurons immunoreactive to an antibody to mature GnRH peptide under conditions maximizing detection of stained neurons was significantly heritable in an unselected control (C) line. Furthermore, a selection line that suppresses reproduction in SD (photoperiod responsive, R) had fewer IR-GnRH neurons than a selection line that maintains reproduction in SD (photoperiod nonresponsive, NR). This supports the hypothesis that genetic variation in characteristics of GnRH neurons themselves may be responsible for the observed phenotypic variation in reproduction in SD. The R and NR lines differ genetically in food intake and iodo-melatonin receptor binding, as well as in other characteristics. The latter findings are consistent with the hypothesis that genetic variation occurs in the nutritional and hormonal inputs to GnRH neurons. Genetic variation also exists in the phenotypic plasticity of responses to two combinations of treatments, (1) food and photoperiod, and (2) photoperiod and age, indicating genetic variation in individual norms of reaction within this population. Overall, the apparent multiple sources of genetic variation within this population suggest that there may be multiple alternative combinations of alleles for both the R and NR phenotypes. If that interpretation is correct, we suggest that this offers some support for the evolutionary "potential" hypothesis and is inconsistent with the evolutionary "constraint" and "symmorphosis" hypotheses for the evolution of complex neuroendocrine pathways.     

Olfactory bulbectomy blocks mating-induced ovulation in musk shrews (Suncus murinus)

In many species, reproductive function can be modified by olfactory inputs. We employed bilateral olfactory bulbectomy (BULBX) to examine the effects of disruption of olfactory inputs on mating behavior and ovulation in female musk shrews. On several measures, sexual behavior was delayed in BULBX females compared to controls. When females were mated on five consecutive days, the majority of unoperated and sham-operated (SHAM) shrews ovulated; only one female subjected to BULBX ovulated. Administration of GnRH induced ovulation in the majority of females. We performed immunocytochemistry to assess the effects of bulbectomy on mating-induced responses of the neural GnRH system. In BULBX and SHAM females, the numbers of cells containing proGnRH immunoreactivity in the medial septum (MS)/diagonal band (DB) were significantly elevated 1 h after mating. Bulbectomy increased the numbers of GnRH-immunoreactive peptide-containing cells in the preoptic area, but it reduced neuron numbers in the MS/DB, as compared with those in SHAM controls. In addition, the GnRH-immunoreactive fiber area in the median eminence was greater in BULBX than in SHAM females. In sum, female musk shrews can display receptivity and engage in copulation without olfactory inputs. However, the olfactory system is essential for mating-induced ovulation.     

Competitive antagonism of nitric oxide synthetase causes weight loss in mice.

These studies demonstrate that the competitive antagonist of nitric oxide synthesis, L-NG-nitro-arginine methyl ester (NO Arg ME), produces an L-arginine reversible decrease in food intake in mice. NO Arg ME also blocked the feeding effect of the potent orexigenic peptide, neuropeptide Y. NO Arg ME produced weight loss when administered over 5 days. The studies suggest that nitric oxide is a physiological modulator of food intake and that nitric oxide synthetase inhibitors may be useful in the management of obesity.     

The emetic activity of staphylococcal enterotoxins,SEK, SEL,SEM, SEN and SEO in a small emetic animal model,the house musk shrew

Staphylococcal enterotoxins (SEs) produced by Staphylococcus aureus are the most recognizable causative agents of emetic food poisoning in humans. New types of SEs and SE‐like (SEl) toxins have been reported. Several epidemiological investigations have shown that the SEs and SEl genes, particularly, SEK, SEL, SEM, SEN and SEO genes, are frequently detected in strains isolated from patients with food poisoning. The purpose of the present study was to evaluate the emetic activity of recently identified SEs using a small emetic animal model, the house musk shrew. The emetic activity of these SEs in house musk shrews was evaluated by intraperitoneal administration and emetic responses, including the number of shrews that vomited, emetic frequency and latency of vomiting were documented. It was found that SEs induce emetic responses in these animals. This is the first time to demonstrate that SEK, SEL, SEM, SEN and SEO possess emetic activity in the house musk shrew.     

Patterns of sexual receptivity in the female musk shrew (Suncus murinus)

Female musk shrews (Suncus murinus) were tested daily to examine patterns of sexual receptivity. When only mounting was used as a criterion (to avoid pregnancy), nonpregnant females remained sexually receptive to males every day for 14 consecutive days. When insemination was allowed, most females continued to copulate for the first 5 days of pregnancy. Receptivity declined markedly around Day 10 of gestation, but a few females were receptive even into late pregnancy. Lactating females copulated with males 5 and 10 days after parturition. In general, unlike most mammals studied in the laboratory, the nonpregnant female musk shrew has no behavioral estrous cycle. Musk shrews are ready to mate anytime except in mild to late pregnancy, and even then occasionally mating is found.     

Evidence that gonadotropin-releasing hormone (GnRH) II stimulates luteinizing hormone and follicle-stimulating hormone secretion from monkey pituitary cultures by activating the GnRH I receptor

Mammalian gonadotropin-releasing hormone (GnRH) I is the neuropeptide that regulates reproduction. In recent years, a second isoform of GnRH, GnRH II, and its highly selective type II GnRH receptor were cloned and identified in monkey brain, but its physiological function remains unknown. We sought to determine whether GnRH II stimulates LH and FSH secretion by activating specific receptors in primary pituitary cultures from male monkeys. Dispersed pituitary cells were maintained in steroid-depleted media and stimulated with GnRH I and/or GnRH II for 6 h. Cells were also treated with Antide (Bachem, King of Prussia, PA), a GnRH I antagonist, to block gonadotropin secretion. In monkey as well as rat pituitary cultures, GnRH II was a less effective stimulator of LH and FSH secretion than was GnRH I. In both cell preparations, Antide completely blocked LH and FSH release provoked by GnRH II as well as GnRH I. Furthermore, the combination of GnRH I and GnRH II was no more effective than either agonist alone. These results indicate that GnRH II stimulates FSH and LH secretion, but they also imply that this action occurs through the GnRH I receptor. The GnRH II receptors may have a unique function in the monkey brain and pituitary other than regulation of gonadotropin secretion.     

Establishment of an outbred strain (Jic: SUN) in the house musk shrew, Suncus murinus]

Planned reproduction of the house musk shrew, Suncus murinus was investigated using 3,016 females for mating. The birth rate and the weaning rate attained were 77.2% and 82.3%, respectively. The average litter size was 3.4. The mean value of the productive index (number of weaned individuals/number of females mated) was estimated as 2.2. The highest value of the productive index, 2.6, was recorded when the fourth litter was used for reproduction. No seasonal fluctuation in the reproductive ability was observed through the course of the present breeding. In the present study, we have maintained the closed colony of the house musk shrew for six years. This colony is established as a outbred strain, and is designated as Jic: SUN.     

Conserved amino acid residues that are important for ligand binding in the type I gonadotropin-releasing hormone (GnRH) receptor are required for high potency of GnRH II at the type II GnRH receptor

GnRH I regulates reproduction. A second form, designated GnRH II, selectively binds type II GnRH receptors. Amino acids of the type I GnRH receptor required for binding of GnRH I (Asp2.61(98), Asn2.65(102), and Lys3.32(121)) are conserved in the type II GnRH receptor, but their roles in receptor function are unknown. We have delineated their functions using mutagenesis, signaling and binding assays, immunoblotting, and computational modeling. Mutating Asp2.61(97) to Glu or Ala, Asn2.65(101) to Ala, or Lys3.32(120) to Gln decreased potency of GnRH II-stimulated inositol phosphate production. Consistent with proposed roles in ligand recognition, mutations eliminated measurable binding of GnRH II, whereas expression of mutant receptors was not decreased. In detailed analysis of how these residues affect ligand-dependent signaling, [Trp2]-GnRH I showed lesser decreases in potency than GnRH I at the Asp2.61(97)Glu mutant. In contrast, [Trp2]-GnRH II showed the same loss of potency as GnRH II at this mutant. This suggests that Asp2.61(97) contributes to recognition of His2 of GnRH I, but not of GnRH II. GnRH II showed a large decrease in potency at the Asn2.65(101)Ala mutant compared with analogs lacking the CO group of Gly10NH2. This suggests that Asn2.65(101) recognizes Gly10NH2 of GnRH II. GnRH agonists showed large decreases in potency at the Lys3.32(120)Gln mutant, but antagonist activity was unaffected. This suggests that Lys3.32(120) recognizes agonists, but not antagonists, as in the type I receptor. These data indicate that roles of conserved residues are similar, but not identical, in the type I and II GnRH receptors.     

Comparative FISH mapping of human cDNA clones to chromosomes of the musk shrew (Suncus murinus, Insectivora)

Forty-one cDNA clones of human functional genes were newly mapped to chromosomes of the musk shrew (Suncus murinus, Insectivora) by fluorescence in situ hybridization, and a comparative cytogenetic map of 51 genes, including 10 genes reported in our previous study, was constructed between human (HSA) and musk shrew (SMU) chromosomes. In this comparative map, the 51 genes localized to human autosomes, except HSA 8, 16, and 20, were mapped to 15 shrew autosomes, except SMU 4, 16, 17 and 18. Twelve conserved segments were identified between human and shrew chromosomes, and six segments among the musk shrew, human, and mouse. Our results defined the presence of at least one inversion and several interchromosomal rearrangements that occurred during evolution after the two species diverged from a common ancestor. Localization of three major histocompatibility complex (MHC) genes to shrew chromosome 3 suggested that the MHC genes of the musk shrew are located in a cluster on chromosome 3. The cytogenetic map constructed in this study is the first cytogenetic map with many functional genes in insectivore species. This approach provides clues for clarifying the chromosomal evolution in this order.     

Evolutionary aspects of gonadotropin-releasing hormone and its receptor

Summary 1. Gonadotropin-releasing hormone (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, during evolution the peptide was subject to gene duplication and structural changes, and multiple molecular forms have evolved.2. Eight variants of GnRH are known, and at least two different forms are expressed in species from all vertebrate classes: chicken GnRH II and a second, unique, GnRH isoform.3. The peptide has been recruited during evolution for diverse 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.4. Evidence suggests that in most species the early-evolved and highly conserved chicken GnRH II has a neurotransmitter function, while the second form, which varies across classes, has a physiologic role in regulating gonadotropin release.5. We review here evolutionary aspects of the family of GnRH peptides and their receptors.