Regulation of luteinizing hormone secretion by neuropeptide Y in rats: hypothalamic and pituitary actions

NPY--an endearing journey in search of a neurochemical on/off switch for appetite, sex and reproduction

Although a dynamic link between the two innate drives, appetite for food and the urge to reproduce, in vertebrate evolution has been known for a long time, a distinct neurochemical pathway mediating this integration has only recently been appreciated. Study of the precise anatomy of the neural track began in the early to mid 20th century after the sites of genesis of the two instincts were localized to the hypothalamus. This report narrates the birth and fruition to maturity of insights into the commonality of hypothalamic neuropeptide Y (NPY) signaling for the two instinctual drives along two distinct pathways.

Regulation of gonadotropin-releasing hormone release by cyclic AMP signalling pathways

The frequency and amplitude of gonadotropin-releasing hormone (GnRH) pulses are tightly regulated for the maintenance of reproductive cycles. Pulsatile GnRH release was shown to be an intrinsic property of murine GT1 GnRH neurons, and primate placodal GnRH neurons. GT1 neurons show spontaneous action potentials that are associated with Ca2+ oscillations and hormone secretion. Increased cyclic AMP (cAMP) levels in GT1 neurons appear to stimulate GnRH release by activation of cAMP-gated cation (CNG) channels. Activation of the CNG channels correlated with increased neuron excitability and Ca2+ oscillations. Activation of protein kinase A is not necessary for cAMP-induced stimulation of GnRH secretion, but appears to activate negative feedback pathways. Potential negative feedback pathways may decrease cAMP levels by inhibiting adenylyl cyclase V, and activating the phosphodiesterase, PDE4D3. These stimulatory and inhibitory cAMP-signalling pathways appear to regulate the excitability of the GT1 neurons, and may constitute a biological clock timing the pulsatile release of GnRH.

Topography and associations of luteinizing hormone-releasing hormone and neuropeptide Y-immunoreactive neuronal systems in the human diencephalon

Neuropeptide Y (NPY) potentiates the effect of luteinizing hormone-releasing hormone (LHRH) on luteinizing hormone secretion in several species, including human. In addition to the pituitary sites, the interactions of the NPY and LHRH systems may involve diencephalic loci. However, the morphologic basis of this putative communication has not yet been elucidated in the human brain. To discover interaction sites, the distribution and connections of LHRH and NPY-immunoreactive (IR) neuronal elements in the human hypothalamus were investigated by means of light microscopic single- and double-label immunocytochemistry. NPY-IR perikarya and fibers were found to be widely distributed in the ventral diencephalon, with high densities in the preopticoseptal, periventricular, and tuberal regions. Small neuronal cell groups were infiltrated with a dense network of varicose NPY-IR fibers in the lateral preoptic area. The LHRH-IR perikarya were located mainly in the preopticoseptal region, diagonal band of Broca, lamina terminalis, and periventricular and infundibular nuclei. A few LHRH-IR neurons and fibers were scattered in the mamillary region. The overlap between the NPY and LHRH systems was apparent in the periventricular, paraventricular, and infundibular nuclei. Double-labeling immunohistochemistry showed NPY-IR axon varicosities in contact with LHRH-IR perikarya and main dendrites. The putative innervation of LHRH neurons by NPY-IR fibers was also seen in 1-microm-thick plastic sections and with confocal laser scanning microscope, thus further supporting the functional impact of NPY-IR terminals on LHRH-IR neurons. The present findings suggest that the hypophysiotropic LHRH-synthesizing neurons may be innervated by intrahypothalamic NPY-IR fibers. Confirmation by ultrastructural analysis would demonstrate that the LHRH system in the human hypothalamus is regulated by NPY, as has been demonstrated in nonhuman species.

Evidence that stimulation of two modalities of pituitary luteinizing hormone release in ovarian steroid-primed ovariectomized rats may involve neuropeptide Y Y1 and Y4 receptors

A large body of evidence indicates that neuropeptide Y (NPY) is involved in stimulation of basal and cyclic release of hypothalamic LHRH and pituitary LH. To identify the NPY receptor subtypes that mediate the excitatory effects of NPY in these two modalities of LH release, we studied the effects of 1229U91, a selective Y1 receptor antagonist and Y4 receptor agonist, in two experimental paradigms that reproduce the two modalities of LH secretion in steroid-primed ovariectomized (OVX) rats. Rats were ovariectomized and implanted with a permanent cannula into the lateral cerebroventricle. In the first experiment, rats received estradiol benzoate (EB, 30 microg/rat) on day 5, followed 2 days later with progesterone (2 mg/rat) at 1000 h to induce an afternoon LH surge. 1229U91 (30 microg/3 microl) or vehicle (control) was injected intracerebroventricularly into these rats either once at 1300 h or twice (15 microg/injection) at 1100 and 1200 h. Blood samples were collected before progesterone injection at 1000 h and at hourly intervals from 1300 -1800 h via an intrajugular cannula implanted on the previous day. In control rats, serum LH levels rose significantly at 1400 h, and these high levels were maintained until 1700 h. After two injections of 1229U91, LH levels displayed a tendency to rise at 1300-1400 h, as in controls, but thereafter, decreased rapidly below the control range. In the second experiment, the acute effect of 1229U91 on LH release was evaluated in OVX rats pretreated with EB alone. Saline alone or saline containing 1, 3, 10, or 30 microg 1229U91 was injected intracerebroventricularly at 1000 h, and the effects on LH release were analyzed at 10, 20, 30, and 60 min. 1229U91 elicited a dose-dependent stimulation of LH release, with maximal response (950% of basal levels) occurring at 10 min after the 30-microg dose; elevated levels were maintained for 1 h. Because 1229U91 is a potent Y4 agonist with some affinity for Y5 receptors, these results raised the possibility that activation of Y4/Y5 receptors by 1229U91 may augment LH release. Therefore, we examined the effects of icv administration of rat pancreatic polypeptide, a Y4-selective agonist, and [D-Trp32]-NPY, a Y5 agonist on LH release in EB-primed rats. Rat pancreatic polypeptide (0.5-2 microg/rat) stimulated LH release in a dose-related manner, and peak levels (280% of basal levels) were seen at 10-20 min; the response evoked by a higher dose (10 microg) was smaller than that induced by 0.5 or 2 microg. [D-Trp32]-NPY was relatively less effective, because only the highest (10-microg) dose elicited a modest stimulation (244% of basal levels). These results demonstrate that 1229U91, a Y1 antagonist and Y4 agonist, evokes two types of responses; it suppresses the protracted ovarian steroid-induced LH surge, and acutely, it also stimulates LH. These results imply that normally two different types of NPY receptors may mediate the stimulation of LH release. Because 1229U91 is a Y1 receptor antagonist, inhibition of the steroid-induced LH surge by 1229U91 suggests that Y1 receptors may mediate the cyclic release of LH. On the other hand, acute stimulation of LH by 1229U91 implies that the Y4 agonist-like activity of 1229U91 may mediate the basal release of LH and that either NPY or a yet-to-be-identified endogenous Y4 receptor agonist may activate Y4 receptors in the hypothalamus to stimulate LH release.

Neuropeptide Y (NPY) actions on the corticotroph cell of the anterior pituitary gland are not mediated by a direct effect

Neuropeptide Y has been implicated in the activation of the hypothalamic-pituitary-adrenal (HPA) axis, the regulation of growth and sexual function, and is the most potent stimulant of feeding yet reported. The actions of NPY on the HPA axis are thought to be mediated via an activation of the corticotrophin releasing hormone (CRH) neurones within the paraventricular nucleus of the hypothalamus. The ability of NPY to directly influence the corticotroph cell is currently controversial. These studies investigated whether NPY could have a direct influence on anterior pituitary adrenocorticotrophic hormone (ACTH) release. In dispersed male rat anterior pituitary cells, NPY (1-1000 nM) had no effect on either basal or CRH (1 nM) stimulated ACTH release. Basal release, NPY (1000 nM) 111 +/- 6% vs. control 103 +/- 5%. CRH stimulated release, CRH (1 nM) with NPY (1000 nM) 226 +/- 23% vs. CRH (1 nM) alone 228 +/- 20%. In addition, NPY (1000 nM) had no effect on either basal or CRH (1 nM) stimulated ACTH release in the mouse corticotroph cell line, AtT-20. Thus, in two models of the anterior pituitary corticotroph NPY had no effect on ACTH release. NPY induced activation of the HPA axis is likely to be mediated via a modulation of hypothalamic CRH and not via a direct action at the level of the anterior pituitary.

Effects of an intravenous injection of NPY on leptin and NPY-Y1 receptor mRNA expression in ovine adipose tissue

Neuropeptide Y (NPY) is highly expressed in hypothalami of undernourished and genetically obese animals, and is a potent regulator of food intake and reproduction. Leptin, a protein expressed by adipocytes, has been reported to reduce hypothalamic NPY expression. We recently detected (by ribonuclease protection assay [RPA]) expression of the NPY receptor subtype Y1 (but not Y2) mRNA in adipose tissue. Based on these observations we hypothesized that NPY-Y1 receptors in adipose may represent a peripheral mechanism by which NPY can regulate leptin expression in a direct and rapid manner. To test this hypothesis, adipose samples were biopsied from the tailhead region of 48 +/- 3 kg wether lambs immediately before and 30 min after a single intravenous injection of 50 micrograms porcine NPY ("treated" animals, n = 5), or vehicle ("control" animals, n = 4). Injection of NPY resulted in an increase in expression (P = 0.013; as measured by RPA) of both leptin and NPY-Y1 mRNA. In treated animals, negative correlations were found between response in leptin expression and body weight (r = -0.82, P = 0.092), and between leptin response and initial leptin mRNA levels (r = -0.81, P = 0.097). These data provide evidence of a peripheral mechanism by which NPY may regulate adipocyte expression of both leptin and NPY-Y1 receptor mRNA.

cDNA cloning and tissue-specific gene expression of ovine leptin, NPY-Y1 receptor, and NPY-Y2 receptor

The physiological regulation of food intake is a critical factor in both the rate at which an animal grows and its reproductive activity. Recently, progress has been made in elucidating a complex system in which insulin, leptin, and neuropeptide Y function to monitor an animal's energy balance and regulate feed intake and fertility. RNA was extracted from ovine hypothalamic, anterior pituitary, pancreas, and adipose tissue. Using the reverse transcription-polymerase chain reaction. cDNAs were cloned and sequenced for leptin (350 base pairs [bp], GenBank accession number U62123 and 441 bp, GenBank accession number U84247), NPY-Y1 receptor (350 bp, GenBank accession no. U62122) and NPY-Y2 receptor (440 bp, GenBank accession no. U83458). Probes generated from these clones were used to detect mRNA expression within tissues thought to be involved in the coregulation of feed intake and reproduction. Leptin was found to be expressed in sheep adipose tissue. The ovine NPY-Y1 receptor mRNA was detected within the arcuate nucleus and paraventricular nucleus of the hypothalamus, the dentate gyrus of the hippocampus and in pancreatic, anterior pituitary, and adipose tissues. Expression of ovine NPY-Y2 receptor mRNA was detected in the hippocampus and within pancreatic tissue. These observations provide evidence of potential mechanisms that exist for mediating communication between peripheral and central tissues within the insulin-leptin-NPY pathway.

Leptin receptor mRNA is expressed in ewe anterior pituitary and adipose tissues and is differentially expressed in hypothalamic regions of well-fed and feed-restricted ewes

Infertility associated with suboptimal nutrition is a major concern among livestock producers. Recently, much effort has been put into understanding the role of the protein leptin in regulating feed intake and reproduction. Leptin, produced by adipocytes, has receptors in the hypothalamus, but more precise locations of leptin receptor-expressing cell bodies have not been reported in a livestock species. The leptin receptor transcript has several splice variants in the mouse and human, but only the "long-form" product (OBRL) is capable of signal transduction. A partial ovine long-form leptin receptor cDNA was cloned and used to evaluate OBRL mRNA expression within hypothalamic, anterior pituitary, and adipose tissues of ovariectomized adult ewes. Expression was detected in reverse transcription-polymerase chain reaction products of all tissues examined. OBRL mRNA was detected by in situ hybridization in the ventromedial and arcuate nuclei of the hypothalamus. In ewes that had been feed restricted for 3 wk before tissue collection, the expression of OBRL mRNA in these areas was greater (P < 0.05) than that found in well-fed ewes. These findings provide evidence that the full-length leptin receptor is expressed in hypothalamic, anterior pituitary, and adipose tissue (the latter proffering an autoregulatory mechanisms for leptin) and that within the hypothalamus, this receptor form is differentially expressed in well-fed vs. feed-restricted animals.

Gonadal hormone feedback on pituitary gonadotropin genes

The pituitary gonadotropins LH and FSH, which act on the ovaries and testes to promote gametogenesis and sex steroid production, are regulated by changes in the levels of steroids and gonadal peptides. Steroid feedback can be positive, as demonstrated by the estrogen and LH surge at ovulation, or negative, as demonstrated by the rise in LH and FSH after gonadectomy or reductions in steroid synthesis. Modulatory effects of the steroids estrogen and testosterone may be mediated directly at the level of the pituitary cells, or by alterations in the release of the hypothalamic releasing factor GnRH. Gonadal peptides, including activin and inhibin, have been shown to have direct effects on pituitary cells to alter FSH synthesis specifically, with no effects on LH. Changes in gonadotropin subunit gene transcription and mRNA levels occur very rapidly and have profound effects on physiological levels of the hormones. In this article, the effects of the gonadal steroids and peptides as modifiers of the rat gonadotropin genes in a subunit specific manner are reviewed, and the physiological implications discussed.

Altered pulsatile gonadotropin signaling in nutritional deficiency in the male

Reproduction cannot occur without adequate nutrition. Diets that are nutritionally inadequate delay and disrupt the pubertal development of the reproductive processes of immature experimental animals and humans, and impair the function of the hypothalamic-pituitary-gonadal axis in adults. Although there is a general understanding of the linkages between nutrition and reproduction, there is a lack of detailed knowledge of the exact mechanisms that couple these two systems. The major effects of malnutrition on the hypothalamic-pituitary-gonadal axis reported in the literature are, for the most part, manifested as reduced gonadotropin secretion. Malnutrition results in decreased circulating gonadotropin concentrations. These changes in the reproductive system are associated with impaired gonadal function and subsequent secondary sex organ atrophy and lead, ultimately, to poor reproduction. Decreased hypothalamic release of gonadotropin-releasing hormone (GnRH) has been proposed as the most important etiologic factor for the fasting-induced suppression of pituitary-testicular function. In the human, hypogonadism and infertility develop in both sexes during chronic malnutrition. Most studies on the effects of malnutrition on the reproductive hormones have been performed in women, perhaps because malnutrition in women is promptly accompanied by amenorrhea, whereas in men hypogonadism develops gradually and becomes clinically evident only during more severe malnutrition. With the advent of sensitive assays for measuring reproductive hormones and of modern computerized methods for analyzing the pulsatile secretion of these hormones, however, the function of the hypothalamic-pituitary-testicular axis has been scrutinized and it has, indeed, been observed that this system is disturbed even during acute malnutrition. Here, we review the effects of malnutrition on reproductive function, especially on the pulsatile pattern of LH secretion, in humans and in experimental animals.

Evidence that hypothalamic neuropeptide Y gene expression increases before the onset of the preovulatory LH surge

Neuropeptide Y (NPY), a 36 amino acid residue peptide, is involved in stimulation of LHRH and LH surges on proestrus and those induced by ovarian steroids in ovariectomized (ovx) rats. Recently, we observed that NPY gene expression in the medial basal hypothalamus (MBH) was increased before the onset of the LH surge in the ovarian steroid-primed ovx rats. Since the ovarian steroidal milieu during the estrous cycle is markedly different from that prevailing after ovarian steroid injections in ovx rats, we evaluated in cycling rats the temporal relationship between MBH preproNPY mRNA levels and the preovulatory LH surge on the day of proestrus and compared that with diestrus II, concomitant with basal LH levels. PreproNPY mRNA levels in the MBH were measured by solution hybridization/RNAse protection assay, using a cRNA probe. On the day of diestrus II, preproNPY mRNA levels changed little between 1000 and 1800 h. Quite unexpectedly, preproNPY mRNA levels at 1000 h on proestrus were similar to diestrus II levels, despite additional exposure to ovarian steroids during this interval. However, from these low levels at 1000 h, the preproNPY mRNA profile displayed a biphasic rise. During the first phase, preproNPY mRNA rose significantly at 1200 h and remained elevated at 1300 and 1400 h concomitant with basal serum LH levels. Thereafter, a second rise in preproNPY mRNA began at 1500 h, peaked rapidly at 1600 h and declined significantly at 1800 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y inhibits pertussis toxin-catalyzed ADP-ribosylation in bovine adrenal chromaffin cell membranes

Evidence is presented that the neuropeptide Y receptor is directly coupled to an inhibitory G protein existing in cultured bovine adrenal chromaffin cell membranes. Pertussis toxin catalyzes the [32P]ADP-ribosylation of a 41 kDa plasma membrane protein. 5'-Guanylylimidodiphosphate inhibited the [32P]ADP labelling of this protein in a dose-dependent manner whereas GTP had no effect. Preincubation of the plasma membranes with high concentrations of neuropeptide Y followed by a brief exposure to a low concentration of 5'-guanylylimidodiphosphate significantly inhibited ADP-ribosylation beyond that observed with 5'-guanylylimidodiphosphate alone. These results suggest that the neuropeptide Y receptor in bovine adrenal chromaffin cells is directly coupled to a 41 kDa PTX substrate (presumably the alpha subunit of an inhibitory G protein).

The distribution of neuropeptide Y and dynorphin immunoreactivity in the brain and pituitary gland of the platyfish, Xiphophorus maculatus, from birth to sexual maturity

Immunoreactive neuropeptide Y and dynorphin have been localized in the brain and pituitary gland of the platyfish, Xiphophorus maculatus, at different ages and stages of development from birth to sexual maturity. Immunoreactive neuropeptide Y was found in perikarya and tracts of the nucleus olfactoretinalis, telencephalon, ventral tegmentum and in the neurohypophysis and in the three regions of the adenohypophysis. Immunoreactive dynorphin was found in nerve tracts in the olfactory bulb and in cells of the pars intermedia and the rostral pars distalis of the pituitary gland.

A selective sexually dimorphic response in the median eminence neuropeptide Y

Recent studies show that neuropeptide Y (NPY) is widely distributed in the hypothalamus and that it stimulates luteinizing hormone-releasing hormone (LHRH) release from the medial basal hypothalamus (MBH) of male and female rats. The neuroendocrine factors that regulate NPY neurosecretion in two sexes are not well understood. We have previously observed that orchidectomy (orch) in male rats decreased and testosterone (T) replacement increased NPY levels selectively in the median eminence (ME), arcuate nucleus (ARC) and ventromedial nucleus (VMN) and the KCl-evoked in vitro release of NPY from the MBH was likewise decreased after orch and restored by testosterone replacement. Now we report that these NPY responses are different in female rats following removal of the ovaries. Ovariectomy (ovx) decreased NPY concentrations in the ARC and VMN and not in the ME. Estrogen replacement restored NPY concentrations in the ARC and VMN, however, in the ME NPY concentration decreased significantly. Further, the KCl-evoked NPY release in vitro from either the MBH or ME of intact and ovx rats was similar. In an additional concurrent comparative study in male and female rats, we observed that of the 6 microdissected hypothalamic sites, NPY levels were approximately 50% less in the ME of intact females than in those of intact male rats. Again, in male rats there was a significant decrease in NPY levels in the ME, ARC and VMN after orch, whereas in female rats, NPY levels in the ARC and VMN, but not in the ME decreased after ovx.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of sexual maturation in male rats by melatonin: evidence linking the mechanism of action to changes in the regulation of hypothalamic neuropeptide y

Activation of gonadotrophin-releasing hormone (GnRHJ pathways is a pivotal event in the process of sexual maturation, however the regulatory influences that precipitate this change and lead to the onset of puberty remain poorly understood. Recent studies indicate that neuropeptide Y (NPY) may participate in the regulation of luteinizing hormone secretion by modulating the pattern of GnRH secretion and by directly altering the pituitary responsiveness to GnRH stimulation. To determine whether NPY plays a role in puberty-associated changes in hypothalamic function, levels of NPY-like immunoreactivity (NPY-IR) were measured in a fragment of the hypothalamus encompassing the median eminence and medial portion of the arcuate nucleus (ME-AN), and also in the remainder of the hypothalamus from male rats of different ages. To identify changes in hypothaiamic NPY linked to the process of sexual development, the effect of delaying sexual maturation by daily afternoon administration of 100 μg melatonin (MT) from 20 to 40 days was investigated. In the hypothalamus and ME-AN, total NPY content increased progressively with age. Expressed as a concentration (fmol/μg extracted protein), peak values for the ME-AN (55.4 ± 7.0) were observed at 30 days of age followed by a decline to lower levels (30.2 ± 1.9) at 40 days. Daily afternoon administration of MT from 20 days of age resulted in significant increases (P<0.01) in the levels of NPY-IR in the ME-AN compared to control values at 30 and 40 days of age. MT was without effect on NPY-IR levels in the remainder of the hypothalamus. When MT was administered in the early morning, a procedure which does not delay sexual maturation, NPY-IR values for the ME-AN region were not different from control rats indicating that the MT-induced changes in NPY were related to the effects on sexual maturation. Using pituitary luteinizing hormone content and seminal vesicle weight as indices of sexual development, significant inverse correlation coefficients (P<0.001) between these parameters and the NPY concentration in the ME-AN were observed (r =-0.79 and -0.70, respectively). From published data it is not possible to conclude whether the main effects of NPY are exerted at the hypothalamic or pituitary level. However, the changes in the NPY content of the ME-AN observed during the onset of puberty, and the influence of MT on these changes, support assertions that NPY is involved in the regulation of sexual maturation.

Hypothalamic neuropeptide Y (NPY) in obese Zucker rats: implications in feeding and sexual behaviors

Neuropeptide Y (NPY), a peptide of the pancreatic polypeptide family, is actually considered to be the most potent stimulator of food intake in rats when centrally injected. It has also suppressive effects on several components of sexual behavior. It was measured in discrete microdissected brain nuclei in obese hyperphagic Zucker fa/fa rats also characterized by a deficient reproductive function, as well as in their lean homozygous (Fa/Fa) and heterozygous (Fa/fa) counterparts. When compared with the lean (Fa/Fa) rats, NPY concentrations were significantly increased in the obese rats in the arcuate nucleus-median eminence (ARCME, +300%), in the paraventricular (PVN, +60%), suprachiasmatic (SCH, +90%), accumbens (+100%) and supraoptic (+40%) nuclei, as well as in the median preoptic area (MPOA, +70%). As PVN is one of the most important nuclei involved in the control of food intake and one site of NPY action, the high levels found in this nucleus might be a major component at the origin of hyperphagia in the obese animals. Food intake might be overstimulated by a sustained production of NPY as shown by the high concentrations found in the ARCME. NPY might also intervene in the pattern of food intake, for NPY contents were also largely modified in the SCH, the nucleus regulating feeding periodicity and in the MPOA, which is possibly involved in the regulation of energy balance. Finally, as the MPOA is the only site of action of NPY on sexual behavior, the higher levels measured in this area might contribute to the defective reproductive function of the obese Zucker fa/fa rat.

Dynamic changes in neuropeptide y concentrations in the median eminence in association with preovulatory luteinizing hormone release in the rat

Abstract Neuropeptide Y (NPY) stimulates luteinizing hormone (LH) release in the rat by a dual action, one in the hypothalamus to excite LH-releasing hormone (LHRH) release and the other at the level of pituitary gonadotrophs to activate and/or potentiate LH release induced by LHRH. Because NPY produces effects similar to norepinephrine with which it may comprise an excitatory hypothalamic circuit, it was hypothesized that NPY concentrations in the hypothalamus would change in a time- and site-specific manner in association with the preovulatory LH surge on proestrus. Concentrations of NPY in individual nuclei of the preoptic-tuberal pathway and serum LH levels were estimated by radioimmunoassays in rats during diestrus 2 and proestrus. On proestrus, serum LH levels were basal between 1000 and 1400 h, rose significantly at 1500 h and plateaued between 1600 and 1800 h. Of the five neural sites examined, only NPY in the median eminence displayed marked fluctuations in close association with the LH surge. NPY concentrations were low between 1000 and 1300 h, and rose abruptly at 1400 h (P<0.05) preceding the onset of LH rise at 1500 h. These elevated levels were maintained until 1600 h, during which time serum LH rose to a plateau and then fell at 1800 h to the low range seen between 1000 and 1300 h. In contrast, the pattern of changes in NPY levels in the arcuate nucleus, suprachiasmatic nucleus and medial preoptic area, three additional sites in the preoptic-tuberal pathway known to participate in the preovulatory LH surge, were markedly different from that seen in the median eminence. In each of these three sites, NPY levels rose significantly at 1800 h from the values at 1000 to 1200 h with a slightly different time-course of increment. None of these areas exhibited changes in NPY concentrations on diestrus 2; NPY concentrations also were unaltered on diestrus 2 or proestrus in the ventromedial nucleus. The present observations of site-specific, dynamic changes in NPY levels on proestrus, in a manner previously documented for LHRH, support the hypothesis that a subset of NPY neurons terminating in the median eminence may be a component of excitatory neural circuitry that either independently or in co-action with the adrenergic system is responsible for the induction of preovulatory LH release.