Inhibitory effect of neuropeptide Y on growth hormone secretion in rats is mediated by both Y1- and Y2-receptor subtypes and abolished after anterolateral deafferentation of the medial basal hypothalamus

Arcuate Nucleus of the Hypothalamus: Anatomy, Physiology, and Diseases

The hypothalamus is part of the diencephalon and has several nuclei, one of which is the arcuate nucleus. The arcuate nucleus of hypothalamus (ARH) consists of neuroendocrine neurons and centrally-projecting neurons. The ARH is the center where the homeostasis of nutrition/metabolism and reproduction are maintained. As such, dysfunction of the ARH can lead to disorders of nutrition/metabolism and reproduction. Here, we review various types of neurons in the ARH and several genetic disorders caused by mutations in the ARH.

Early overnutrition sensitizes the growth hormone axis to the impact of diet-induced obesity via sex-divergent mechanisms

In addition to its essential role in the physiological control of longitudinal growth, growth-hormone (GH) is endowed with relevant metabolic functions, including anabolic actions in muscle, lipolysis in adipose-tissue and glycemic modulation. Adult obesity is known to negatively impact GH-axis, thereby promoting a vicious circle that may contribute to the exacerbation of the metabolic complications of overweight. Yet, to what extent early-overnutrition sensitizes the somatotropic-axis to the deleterious effects of obesity remains largely unexplored. Using a rat-model of sequential exposure to obesogenic insults, namely postnatal-overfeeding during lactation and high-fat diet (HFD) after weaning, we evaluated in both sexes the individual and combined impact of these nutritional challenges upon key elements of the somatotropic-axis. While feeding HFD per se had a modest impact on the adult GH-axis, early overnutrition had durable effects on key elements of the somatotropic-system, which were sexually different, with a significant inhibition of pituitary gene expression of GH-releasing hormone-receptor (GHRH-R) and somatostatin receptor-5 (SST5) in males, but an increase in pituitary GHRH-R, SST2, SST5, GH secretagogue-receptor (GHS-R) and ghrelin expression in females. Notably, early-overnutrition sensitized the GH-axis to the deleterious impact of HFD, with a significant suppression of pituitary GH expression in both sexes and lowering of circulating GH levels in females. Yet, despite their similar metabolic perturbations, males and females displayed rather distinct alterations of key somatotropic-regulators/ mediators. Our data document a synergistic effect of postnatal-overnutrition on the detrimental impact of HFD-induced obesity on key elements of the adult GH-axis, which is conducted via mechanisms that are sexually-divergent.

Neuropeptide Y: An Anti-Aging Player?

Accumulating evidence suggests that neuropeptide Y (NPY) has a role in aging and lifespan determination. In this review, we critically discuss age-related changes in NPY levels in the brain, together with recent findings concerning the contribution of NPY to, and impact on, six hallmarks of aging, specifically: loss of proteostasis, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing, cellular senescence, and mitochondrial dysfunction. Understanding how NPY contributes to, and counteracts, these hallmarks of aging will open new avenues of research on limiting damage related to aging.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

The neuropeptide Y-ergic system: potential therapeutic target against bone loss with obesity treatments

Obesity is no longer considered to provide protection against osteoporosis. Moreover, treatments for obesity are now suspected of reducing bone mass. With the escalating incidence of obesity, combined with increases in the frequency, duration and intensity of interventions used to combat it, we face a potential increase in health burden due to osteoporotic fractures. The neuropeptide Y-ergic system offers a potential target for the prevention and anabolic treatment of bone loss in obesity, due to its dual role in the regulation of energy homeostasis and bone mass. Although the strongest stimulation of bone mass by this system appears to occur via indirect hypothalamic pathways involving Y2 receptors (one of the five types of receptors for neuropeptide Y), Y1 receptors on osteoblasts (bone-forming cells) induce direct effects to enhance bone mass. This latter pathway may offer a suitable target for anti-osteoporotic treatment while also minimizing the risk of adverse side effects.

Actions of NPY, and its Y1 and Y2 receptors on pulsatile growth hormone secretion during the fed and fasted state

The hypothalamic NPY system plays an important role in regulating food intake and energy expenditure. Different biological actions of NPY are assigned to NPY receptor subtypes. Recent studies demonstrated a close relationship between food intake and growth hormone (GH) secretion; however, the mechanism through which endogenous NPY modulates GH release remains unknown. Moreover, conclusive evidence demonstrating a role for NPY and Y-receptors in regulating the endogenous pulsatile release of GH does not exist. We used genetically modified mice (germline Npy, Y1, and Y2 receptor knock-out mice) to assess pulsatile GH secretion under both fed and fasting conditions. Deletion of NPY did not impact fed GH release; however, it reversed the fasting-induced suppression of pulsatile GH secretion. The recovery of GH secretion was associated with a reduction in hypothalamic somatotropin release inhibiting factor (Srif; somatostatin) mRNA expression. Moreover, observations revealed a differential role for Y1 and Y2 receptors, wherein the postsynaptic Y1 receptor suppresses GH secretion in fasting. In contrast, the presynaptic Y2 receptor maintains normal GH output under long-term ad libitum-fed conditions. These data demonstrate an integrated neural circuit that modulates GH release relative to food intake, and provide essential information to address the differential roles of Y1 and Y2 receptors in regulating the release of GH under fed and fasting states.

A comparative review of short and long neuropeptide F signaling in invertebrates: Any similarities to vertebrate neuropeptide Y signaling?

Neuropeptides referred to as neuropeptide F (NPF) and short neuropeptide F (sNPF) have been identified in numerous invertebrate species. Sequence information has expanded tremendously due to recent genome sequencing and EST projects. Analysis of sequences of the peptides and prepropeptides strongly suggest that NPFs and sNPFs are not closely related. However, the NPFs are likely to be ancestrally related to the vertebrate family of neuropeptide Y (NPY) peptides. Peptide diversification may have been accomplished by different mechanisms in NPFs and sNPFs; in the former by gene duplications followed by diversification and in the sNPFs by internal duplications resulting in paracopies of peptides. We discuss the distribution and functions of NPFs and their receptors in several model invertebrates. Signaling with sNPF, however, has been investigated mainly in insects, especially in Drosophila. Both in invertebrates and in mammals NPF/NPY play roles in feeding, metabolism, reproduction and stress responses. Several other NPF functions have been studied in Drosophila that may be shared with mammals. In Drosophila sNPFs are widely distributed in numerous neurons of the CNS and some gut endocrines and their functions may be truly pleiotropic. Peptide distribution and experiments suggest roles of sNPF in feeding and growth, stress responses, modulation of locomotion and olfactory inputs, hormone release, as well as learning and memory. Available data indicate that NPF and sNPF signaling systems are distinct and not likely to play redundant roles.

Concurrent occurrence of chronic lymphocytic thyroiditis with hypothyroidism and growth hormone deficiency in a Turner's syndrome patient

BACKGROUND

Concurrent deficiencies of three hormones in patients with Turner's syndrome (TS) have rarely been reported. Here, we describe a case of a young girl who had Turner's syndrome with concomitant chronic lymphocytic thyroiditis, growth hormone deficiency, and hypothyroidism with cardiopericarditis.

CASE

An 11-year-old girl was referred to the outpatient clinic because of short stature, ochriasis, and cardiopalmus. Her ultrasound revealed absence of ovarian tissue. Karyotype examination suggested Turner's syndrome with sex hormone deficiency. She was found to have an abnormal thyroid gland and elevated thyroid stimulating hormone (TSH). A positive thyroid autoantibody titer confirmed the diagnosis of chronic lymphocytic thyroiditis with hypothyroidism. Furthermore, her growth hormone levels were well below normal.

DIAGNOSIS

A multi-endocrine disorder, i.e., Turner's syndrome with chronic lymphocytic thyroiditis, growth hormone deficiency, and hypothyroidism with cardiopericarditis was diagnosed. Growth hormone and thyroxin substitution therapy was suggested.

Understanding the multifactorial control of growth hormone release by somatotropes: lessons from comparative endocrinology

Control of postnatal growth is the main, but not the only, role for growth hormone (GH) as this hormone also contributes to regulating metabolism, reproduction, immunity, development, and osmoregulation in different species. Likely owing to this variety of group-specific functions, GH production is differentially regulated across vertebrates, with an apparent evolutionary trend to simplification, especially in the number of stimulatory factors governing substantially GH release. Thus, teleosts exhibit a multifactorial regulation of GH secretion, with a number of factors, from the newly discovered fish GH-releasing hormone (GHRH) to pituitary adenylate cyclase-activating peptide (PACAP) but also gonadotropin-releasing hormone, dopamine, corticotropin-releasing hormone, and somatostatin(s) directly controlling somatotropes. In amphibians and reptiles, GH secretion is primarily stimulated by the major hypothalamic peptides GHRH and PACAP and inhibited by somatostatin(s), while other factors (ghrelin, thyrotropin-releasing hormone) also influence GH release. Finally, in birds and mammals, primary control of GH secretion is exerted by a dual interplay between GHRH and somatostatin. In addition, somatotrope function is modulated by additional hypothalamic and peripheral factors (e.g., ghrelin, leptin, insulin-like growth factor-I), which together enable a balanced integration of feedback signals related to processes in which GH plays a relevant regulatory role, such as metabolic and energy status, reproductive, and immune function. Interestingly, in contrast to the high number of stimulatory factors impinging upon somatotropes, somatostatin(s) stand(s) as the main primary inhibitory regulator(s) for this cell type.

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Severity of the catabolic condition differentially modulates hypothalamic expression of growth hormone-releasing hormone in the fasted mouse: potential role of neuropeptide Y and corticotropin-releasing hormone

To determine whether the severity of the catabolic condition differentially regulates the GH axis, male mice were either fed ad libitum or fasted for 12, 24, and 48 h. Hypothalami, pituitaries, and stomachs were collected for assessment of mRNA levels by quantitative real-time RT-PCR, and blood collected for measurement of plasma hormone and metabolite levels by commercial assay kits. Overnight (12 h) fasting resulted in a significant suppression of circulating glucose, insulin, IGF-I, and leptin levels and an increase in corticosterone, free fatty acids, and n-octanoyl ghrelin levels, and these directional changes were maintained at the 24- and 48-h time points. Fasting (24 h) also increased circulating GH levels, which was associated with an increase in pituitary mRNA levels for GHRH receptor and ghrelin receptor and a decrease in mRNA levels for somatostatin (SST) receptor (SSTR) subtypes, SSTR2, SSTR3, and SSTR5, where the changes in ghrelin receptor and SSTR expression persisted after 48 h fasting. Hypothalamic SST mRNA levels were not altered by fasting, whereas there was a transient rise in stomach SST mRNA levels 24 h after food withdrawal. In contrast, there was a biphasic effect of fasting on GHRH expression. GHRH mRNA levels were significantly elevated at 12 and 24 h but fell to approximately 50% of fed controls 48 h after food withdrawal. A sequential rise in hypothalamic neuropeptide Y (NPY) and CRH mRNA levels preceded the fall in GHRH expression, where fasting-induced changes in CRH and GHRH mRNA levels were not observed in 48-h-fasted NPY knockout mice. These observations, in light of previous reports showing both NPY and CRH can inhibit GHRH expression and GH release, suggest that these neuronal systems may work in concert to control the ultimate impact of fasting on GH axis function.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

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

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

Expression analysis of hypothalamic and pituitary components of the growth hormone axis in fasted and streptozotocin-treated neuropeptide Y (NPY)-intact (NPY+/+) and NPY-knockout (NPY-/-) mice

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY(+/+)) and NPY-knockout (NPY(-/-)) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY(+/+) and NPY(-/-) did not differ. In the NPY(+/+) mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY(-/-) mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY(-/-) mouse. Induction of diabetes in NPY(+/+) mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY(+/+) and NPY(-/-) mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY(-/-) mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

Putative GH pulse renewal: periventricular somatostatinergic control of an arcuate-nuclear somatostatin and GH-releasing hormone oscillator

Growth hormone (GH) pulsatility requires periventricular-nuclear somatostatin(SRIF(PeV)), arcuate-nuclear (ArC) GH-releasing hormone (GHRH), and systemic GH autofeedback. However, no current formalism interlinks these regulatory loci in a manner that generates self-renewable GH dynamics. The latter must include in the adult rat 1) infrequent volleys of high-amplitude GH peaks in the male, 2) frequent discrete low-amplitude GH pulses in the female, 3) disruption of the male pattern by severing SRIF(PeV) outflow to ArC, 4) stimulation of GHRH and GH secretion by central nervous system delivery of SRIF, 5) inhibition of GH release by central exposure to GHRH, and 6) a reboundlike burst of GHRH secretion induced by stopping peripheral infusion of SRIF. The present study validates by computer-assisted simulations a simplified ensemble formulation that predicts each of the foregoing six outcomes, wherein 1) blood-borne GH stimulates SRIF(PeV) secretion after a long time latency, 2) SRIF(PeV) inhibits both pituitary GH and ArC GHRH release, 3) ArC GHRH and SRIF(ArC) oscillate reciprocally with brief time delay, and 4) SRIF(PeV) represses and disinhibits the putative GHRH-SRIF(ArC) oscillator. According to the present analytic construction, time-delayed feedforward and feedback signaling among SRIF(PeV), ArC GHRH, and SRIF(ArC) could endow the complex physiological patterns of GH secretion in the male and female.

Links between the appetite regulating systems and the neuroendocrine hypothalamus: lessons from the sheep

The hypothalamus is integral to the regulation of energy homeostasis and the secretion of hormones from the pituitary gland. Consequently, hypothalamic systems may have a dual purpose in regulating both neuroendocrine function and appetite. To date, most studies investigating the interface between appetite and hormone secretion have been performed in rats or mice that have been acutely fasted or baring a genetic abnormality causing either obesity or aphagia. By contrast, various physiological models, including chronic food-restriction or photoperiodically driven changes in voluntary food intake, add further perspective to the issue. In this regard, sheep provide an innovative model whereby long-term changes in body weight or extended feeding rhythms can be investigated. This review compares and contrasts data obtained in different species with regard to the neuroendocrinology of appetite, and discusses the benefits and knowledge gained from using various nonrodent models with a particular emphasis on a ruminant species.

Gastric distension-induced pyloric relaxation: central nervous system regulation and effects of acute hyperglycaemia in the rat

1. The pylorus plays an important role in the regulation of gastric emptying. In addition to the autonomic neuropathy associated with long-standing diabetes, acute hyperglycaemia per se has effects on gastric emptying. In this study, the role of the central nervous system in modulating the effects of hyperglycaemia on gastric distension-induced pyloric relaxation was investigated. 2. Gastric distension-induced pyloric relaxation was significantly reduced by subdiaphragmatic vagotomy, hexamethonium (20 mg kg(-1)) and N (G)-nitro-L-arginine methyl ester (L-NAME; 10 mg kg(-1)), a nitric oxide synthase (NOS) biosynthesis inhibitor, in anaesthetized rats. In contrast, neither splanchnectomy nor guanethidine (5 mg kg(-1)) had an effect. 3. An intravenous (I.V.) infusion of D-glucose (20 %) for 30 min, which increased blood glucose concentrations from 5.4 to 12.8 mM, significantly inhibited gastric distension-induced pyloric relaxation. 4. An intracerebroventricular (I.C.V.) injection of D-glucose (3 micromol) also significantly inhibited gastric distension-induced pyloric relaxation without affecting peripheral blood glucose concentrations. 5. I.V. infusion of D-glucose significantly elevated hypothalamic neuropeptide Y (NPY) concentrations. 6. Intracerebroventricular (I.C.V.) administration of NPY (0.03--3 nmol) and a Y1 receptor agonist, [leu(31), pro(34)] NPY (0.03--3 nmol), significantly inhibited gastric distension-induced pyloric relaxation in a dose-dependent manner. 7. I.C.V. administration of a Y1 receptor antagonist, BIBP 3226 (30 nmol), and of a NPY antibody (titre 1:24 000, 3 microl) abolished the inhibitory effects of hyperglycaemia on gastric distension-induced pyloric relaxation. 8. Taken together, these findings suggest that gastric distension-induced pyloric relaxation is mediated via a vago-vagal reflex and NO release. Acute hyperglycaemia stimulates hypothalamic NPY release, which, acting through the Y1 receptor, inhibits gastric distension-induced pyloric relaxation in rats exposed to acute elevations in blood glucose concentrations.

Neuroregulation of growth hormone secretion in domestic animals

Growth hormone (GH) is essential for postnatal somatic growth, maintenance of lean tissue at maturity in domestic animals and milk production in cows. This review focuses on neuroregulation of GH secretion in domestic animals. Two hormones principally regulate the secretion of GH: growth hormone-releasing hormone (GHRH) stimulates, while somatostatin (SS) inhibits the secretion of GH. A long-standing hypothesis proposes that alternate secretion of GHRH and SS regulate episodic secretion of GH. However, measurement of GHRH and SS in hypophysial-portal blood of unanesthetized sheep and swine shows that episodic secretion of GHRH and SS do not account for all episodes of GH secreted. Furthermore, the activity of GHRH and SS neurons decreases after steers have eaten a meal offered for a 2-h period each day (meal-feeding) and this corresponds with reduced secretion of GH. Together, these data suggest that other factors also regulate the secretion of GH. Several neurotransmitters have been implicated in this regard. Thyrotropin-releasing hormone, serotonin and gamma-aminobutyric acid stimulate the secretion of GH at somatotropes. Growth hormone releasing peptide-6 overcomes feeding-induced refractoriness of somatotropes to GHRH and stimulates the secretion of GHRH. Norepinephrine reduces the activity of SS neurons and stimulates the secretion of GHRH via alpha(2)-adrenergic receptors. N-methyl-D,L-aspartate and leptin stimulate the secretion of GHRH, while neuropeptide Y stimulates the secretion of GHRH and SS. Activation of muscarinic receptors decreases the secretion of SS. Dopamine stimulates the secretion of SS via D1 receptors and inhibits the secretion of GH from somatotropes via D2 receptors. Thus, many neuroendocrine factors regulate the secretion of GH in livestock via altering secretion of GHRH and/or SS, communicating between GHRH and SS neurons, or acting independently at somatotropes to coordinate the secretion of GH.

Growth hormone inhibits growth hormone secretion from the rainbow trout pituitary in vitro

Growth hormone (GH) secretion in salmonids and other fish is under the control of a number of hypothalamic factors, but negative feed-back regulation by circulating hormones can also be of importance for the regulation of GH secretion. Mammalian studies show that GH has a negative feed-back effect on its own secretion. In order to elucidate if GH levels present a direct ultra-short negative feedback loop at the pituitary level GH secretion was studied in intact pituitaries from 50 g fish in an in vitro perifusion system. Following an initial equilibrium period pituitaries were exposed to five increasing concentrations (1-1,000 ng ml(-1)) of ovine GH (oGH) in 20-min steps, before being returned to a GH-free perifusion. Ovine GH caused a significant dose-dependent inhibition of GH secretion and it is concluded that GH can exert a direct negative feedback control on GH secretion at the pituitary level.

cNeuropeptide Y family of peptides: Structure, anatomical expression, function, and molecular evolution

Evolutionary relationships between neuroendocrine peptides are often difficult to resolve across divergent phyla due to independent duplication events in different lineages. Thanks to peptide purification and molecular cloning in many different species, the situation is beginning to clear for the neuropeptide Y (NPY) family, which also includes peptide YY (PYY), the tetrapod pancreatic polypeptide (PP) and the fish pancreatic peptide Y (PY). It has long been assumed that the first duplication to occur in vertebrate evolution generated NPY and PYY, as both of these are found in all gnathostomes as well as lamprey. Evidence from other gene families show that this duplication was probably a chromosome duplication event. The origin of a second PYY peptide found in lamprey remains to be explained. Our recent cloning of NPY, PYY and PY in the sea bass proves that fish PY is a separate gene product. We favour the hypothesis that PY is a duplicate of the PYY gene and that it may have occurred late in fish evolution, as PY has so far only been found in acanthomorph fishes. Thus, this duplication seems to be independent of the one that generate PP from PYY in tetrapods, although both tetrapod PP and fish PY are expressed in the pancreas. Studies in the sea bass and other fish show that PY, in contrast to PP, is expressed in the nervous system. We review the literature on the distribution and functional aspects of the various NPY-family peptides in vertebrates.

Key words: neuropeptide Y, pancreatic polypeptide, fish pancreatic peptide, gene duplication.

The effect of growth hormone secretagogues and neuropeptide Y on hypothalamic hormone release from acute rat hypothalamic explants

Growth hormone (GH) secretagogues (GH-releasing peptides and their non-peptide analogues) stimulate growth hormone release via specific G-protein coupled receptors both directly from the pituitary gland and through stimulation of the hypothalamus. The exact mechanism of action in the hypothalamus is not known. The presence of endogenous GH releasing hormone (GHRH) seems to be necessary for the in-vivo actions of growth hormone secretagogues (GHSs), but data suggest that further factors must be involved as well. The effect of GHSs is not entirely specific for the GH axis; they release prolactin and stimulate the hypothalamo-pituitary-adrenal axis causing elevations in circulating ACTH and cortisol levels in both animal and human studies. Recently, it has also been suggested that GHSs stimulate hypothalamic neuropeptide Y (NPY) neurones. In the present study, we have therefore investigated the direct effect of several GHSs (GHRP-6, hexarelin and the non-peptide analogues L-692, 429 and L-692, 585) on GHRH, somatostatin (SS), corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) release in vitro in an acute rat hypothalamic incubation system. We also assessed the effect of NPY on GHRH, SS and AVP release. Freshly removed hypothalami were incubated in control media for 20 min and then in 1-4 consecutive 20-min periods in each of the test substances at different concentrations. There was no significant change in either the basal or potassium-stimulated release of GHRH or SS at low concentrations of any of the secretagogues; however, at millimolar doses a paradoxical inhibition of GHRH was observed with GHRP-6, hexarelin and L-692 585 (data are expressed as the ratio of treated to preceding basal release; at 20 min control group: 0.97+/-0.02, GHRP-6: 0.55+/-0.04, P<0.001 compared to control group; hexarelin: 0. 56+/-0.06, P<0.001, L-692,585: 0.70+/-0.03, P<0.001), while SS was stimulated after 60 or 80 min (at 80 min control: 0.80+/-0.03, hexarelin: 1.23+/-0.07, P<0.05 and L-692,585: 1.37+/-0.11, P<0.05). GHSs stimulated hypothalamic AVP release (at 20 min control: 0. 99+/-0.06 ratio to basal release, 10-4 M concentration of GHRP-6: 6. 31+/-1, P<0.001, hexarelin: 1.88+/-0.4, P<0.01, L-692,429: 1.90+/-0. 5, P<0.05 and L-692,585: 2.34+/-0.96, P<0.01), while no stimulatory effect was found on CRH release. NPY significantly stimulated SS and inhibited basal and potassium-stimulated GHRH release, while potentiating potassium-evoked AVP secretion. The Y1 receptor antagonist BIBP 3226 did not inhibit the effects of NPY on SS, GHRH or AVP release. We therefore conclude that, in this in-vitro rat hypothalamic incubation model, growth hormone secretagogues stimulate the release of AVP but have no effect on either GHRH, SS or CRH at low doses; at high doses paradoxically they inhibit the hypothalamic GH axis similar to in-vivo data in the rat. We speculate that these effects might be mediated by NPY.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.