Effect of parenteral administration of selected nutrients and central injection of gamma-globulin from antiserum to neuropeptide Y on growth hormone secretory pattern in food-deprived rats

A key role for neuropeptide Y in lifespan extension and cancer suppression via dietary restriction

Knowledge of genes essential for the life-extending effect of dietary restriction (DR) in mammals is incomplete. In this study, we found that neuropeptide Y (Npy), which mediates physiological adaptations to energy deficits, is an essential link between DR and longevity in mice. The lifespan-prolonging effect of lifelong 30% DR was attenuated in Npy-null mice, as was the effect on the occurrence of spontaneous tumors and oxidative stress responses in comparison to wild-type mice. In contrast, the physiological processes activated during adaptation to DR, including inhibition of anabolic signaling molecules (insulin and insulin-like growth factor-1), modulation of adipokine and corticosterone levels, and preferential fatty acid oxidation, were unaffected by the absence of Npy. These results suggest a key role for Npy in mediating the effects of DR. We also provide evidence that most of the physiological adaptations to DR could be achieved in mice without Npy.

Cross-talk between orexins (hypocretins) and the neuroendocrine axes (hypothalamic-pituitary axes)

Lesioning and electrical stimulation experiments carried out during the first half of the twentieth century showed that the lateral hypothalamic area (LHA) is involved in the neuroendocrine control of hormone secretion. However, the molecular basis of this phenomenon remained unclear until fifty years later when in 1998, two different laboratories discovered a new family of hypothalamic neuropeptides, the orexins or hypocretins (OX-A/Hcrt1 and OX-B/Hcrt2). Since then, remarkable evidence has revealed that orexins/hypocretins play a prominent role in regulating virtually all the neuroendocrine axes, acting as pivotal signals in the coordination of endocrine responses with regards to sleep, arousal and energy homeostasis. The clinical relevance of these actions is supported by human data showing impairment of virtually all the neuroendocrine axes in orexin/hypocretin-deficient narcoleptic patients. Here, we summarize more than ten years of knowledge about the orexins/hypocretins with particular focus on their role as neuroendocrine regulators. Understanding this aspect of orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and endocrine pathologies.

Orexins (hypocretins) actions on the GHRH/somatostatin-GH axis

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system that includes two major hypothalamic regulators, namely GH-releasing hormone (GHRH) and somatostatin (SST) that stimulate and inhibit, respectively, GH release. Classical experiments involving damage and electrical stimulation suggested that the lateral hypothalamic area (LHA) modulated the somatotropic axis, but the responsible molecular mechanisms were unclear. Evidence obtained during the last decade has demonstrated that orexins/hypocretins, a family of peptides expressed in the LHA controlling feeding and sleep, play an important regulatory role on GH, by inhibiting its secretion modulating GHRH and SST neurones. Considering that GH release is closely linked to the sleep-wake cycle and feeding state, understanding orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and GH-related pathologies, such as GH deficiency.

Acute administration of docosahexaenoic acid increases resistance to pentylenetetrazol-induced seizures in rats

OBJECTIVE

Docosahexaenoic acid (DHA), an omega-3 fatty acid, has been reported to raise seizure thresholds. The purpose of the present study was to test the acute anticonvulsant effects of unesterified DHA in rats, using the maximal pentylenetetrazol (PTZ) seizure model, and also to examine DHA incorporation and distribution into blood serum total lipids and brain phospholipids and unesterified fatty acids. Sedation was measured to monitor for the potential toxicity of DHA.

METHODS

Male Wistar rats received subcutaneous injections of saline, oleic acid (OA), or DHA. An initial pilot study (Experiment 1) established 400mg/kg as an effective dose of DHA in the maximal PTZ seizure test. A subsequent time-response study, using 400mg/kg (Experiment 2), established 1 hour as an effective postinjection interval for administering DHA subcutaneously. A final study (Experiment 3) comprised two different groups. The first group ("seizure-tested rats") received saline, OA, or DHA (400mg/kg) subcutaneously, and were seizure tested in the maximal PTZ test 1 hour later to confirm the seizure latency measurements at that time. The second group ("assay rats") received identical subcutaneous injections of saline, OA, or DHA (400mg/kg). One hour postinjection, however, they were sacrificed for assay rather than being seizure tested. Assays involved the analysis of serum and brain DHA. Sedation was measured in both Experiment 3 groups during the 1-hour period prior to seizure testing or sacrifice.

RESULTS

As noted above, 400mg/kg proved to be an effective subcutaneous dose of DHA (Experiment 1), and 1 hour proved to be the most effective injection-test interval (Experiment 2). In Experiment 3, in the seizure-tested animals, subcutaneous administration of 400mg/kg of DHA significantly increased latency to PTZ seizure onset 1 hour postinjection relative to the saline- and OA-injected controls, which did not differ significantly from each other (P>0.05). In the assay animals, no significant effects of treatment on blood serum total lipids or on brain phospholipid or unesterified fatty acid profiles (P>0.05) were observed. There were also no differences in sedation among the three groups (P>0.05).

CONCLUSION

DHA increases resistance to PTZ-induced seizures without altering measures of sedation and, apparently, without changing DHA concentrations in serum or brain.

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.

Neuropeptide Y modulates effects of bradykinin and prostaglandin E2 on trigeminal nociceptors via activation of the Y1 and Y2 receptors

BACKGROUND AND PURPOSE

Although previous studies have demonstrated that neuropeptide Y (NPY) modulates nociceptors, the relative contributions of the Y1 and Y2 receptors are unknown. Therefore, we evaluated the effect of Y1 and Y2 receptor activation on nociceptors stimulated by bradykinin (BK) and prostaglandin E2 (PGE2).

EXPERIMENTAL APPROACH

Combined immunohistochemistry (IHC) with in situ hybridization (ISH) demonstrated that Y1- and Y2-receptors are collocated with bradykinin (2) (B2)-receptors in rat trigeminal ganglia (TG). The relative functions of the Y1 and Y2 receptors in modulating BK/PGE2-evoked CGRP release and increased intracellular calcium levels in cultured TG neurons were evaluated.

KEY RESULTS

The Y1 and Y2 receptors are co-expressed with B2 in TG neurons, suggesting the potential for direct NPY modulation of BK responses. Pretreatment with the Y1 agonist [Leu31,Pro34]-NPY, inhibited BK/PGE2-evoked CGRP release. Conversely, pretreatment with PYY(3-36), a Y2 agonist, increased BK/PGE2 evoked CGRP release. Treatment with NPY evoked an overall inhibitory effect, although of lesser magnitude. Similarly, [Leu31,Pro34]-NPY inhibited BK/PGE2-evoked increases in intracellular calcium levels whereas PYY(3-36) increased responses. NPY inhibition of BK/PGE2-evoked release of CGRP was reversed by the Y1 receptor antagonist, BIBO3304, and higher concentrations of BIBO3304 significantly facilitated CGRP release. The Y2 receptor antagonist, BIIE0246, enhanced the inhibitory NPY effects.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that NPY modulation of peptidergic neurons is due to net activation of inhibitory Y1 and excitatory Y2 receptor systems. The relative expression or activity of these opposing receptor systems may mediate dynamic responses to injury and pain.

The effect of agouti-related protein on growth hormone secretion in adult male rats

Agouti-related protein (AGRP) and neuropeptide Y (NPY) are synthesized in the same neurons in the hypothalamic arcuate nucleus. We have previously shown that NPY/AGRP neurons contain growth hormone (GH) receptor mRNA, and are activated following systemic GH administration. We also reported that NPY inhibits GH secretion when administered centrally. In this study, we have examined the effect of AGRP on GH secretion. Central administration of AGRP (83-132) as a single injection of 1 or 10 microg/rat, or chronic treatment of 1 microg/rat, every 12 h for 7 days, did not alter the GH secretory pattern of adult male rats. AGRP (83-132) at doses of 1-100 nM (4 h) did not alter baseline- and GHRH-induced GH secretion from the rat pituitary cell cultures. These results suggest that AGRP does not play a significant role in the feedback regulation of the GH secretion.

Nutritional status in the neuroendocrine control of growth hormone secretion: the model of anorexia nervosa

Growth hormone (GH) plays a key role not only in the promotion of linear growth but also in the regulation of intermediary metabolism, body composition, and energy expenditure. On the whole, the hormone appears to direct fuel metabolism towards the preferential oxidation of lipids instead of glucose and proteins, and to convey the energy derived from metabolic processes towards the synthesis of proteins. On the other hand, body energy stores and circulating energetic substrates take an important part in the regulation of somatotropin release. Finally, central and peripheral peptides participating in the control of food intake and energy expenditure (neuropeptide Y, leptin, and ghrelin) are also involved in the regulation of GH secretion. Altogether, nutritional status has to be regarded as a major determinant in the regulation of the somatotropin-somatomedin axis in animals and humans. In these latter, overweight is associated with marked impairment of spontaneous and stimulated GH release, while acute dietary restriction and chronic undernutrition induce an amplification of spontaneous secretion together with a clear-cut decrease in insulin-like growth factor I (IGF-I) plasma levels. Thus, over- and undernutrition represent two conditions connoted by GH hypersensitivity and GH resistance, respectively. Anorexia nervosa (AN) is a psychiatric disorder characterized by peculiar changes of the GH-IGF-I axis. In these patients, low circulating IGF-I levels are associated with enhanced GH production rate, highly disordered mode of somatotropin release, and variability of GH responsiveness to different pharmacological challenges. These abnormalities are likely due not only to the lack of negative IGF-I feedback, but also to a primary hypothalamic alteration with increased frequency of growth hormone releasing hormone discharges and decreased somatostatinergic tone. Given the reversal of the above alterations following weight recovery, these abnormalities can be seen as secondary, and possibly adaptive, to nutritional deprivation. The model of AN may provide important insights into the pathophysiology of GH secretion, in particular as regards the mechanisms whereby nutritional status effects its regulation.

Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding proteinalpha (C/EBPalpha) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBPalpha auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Recombinant porcine leptin reduces feed intake and stimulates growth hormone secretion in swine

Two experiments (EXP) were conducted to test the hypothesis that porcine leptin affects GH, insulin-like growth factor-I (IGF-I), insulin, thyroxine (T4) secretion, and feed intake. In EXP I, prepuberal gilts received intracerebroventricular (i.c.v.) leptin injections. Blood was collected every 15 min for 4 hr before and 3 hr after i.c.v. injections of 0.9% saline (S; n = 3), 10 micrograms (n = 4), 50 micrograms (n = 4), or 100 micrograms (n = 4) of leptin in S. Pigs were fed each day at 0800 and 1700 hr over a 2-wk period before the EXP. On the day of the EXP, pigs were fed at 0800 hr and blood sampling started at 0900 h. After the last sample was collected, feeders were placed in all pens. Feed intake was monitored at 4, 20, and 44 hr after feed presentation. In EXP II, pituitary cells from prepuberal gilts were studied in primary culture to determine if leptin affects GH secretion at the level of the pituitary. On Day 4 of culture, 10(5) cells/well were challenged with 10(-12), 10(-10), 10(-8), or 10(-6) M [Ala15]-h growth hormone-releasing factor-(1-29)NH2 (GRF), 10(-14), 10(-13), 10(-12), 10(-11), 10(-10), 10(-9), 10(-8), 10(-7), or 10(-6) M leptin individually or in combinations with 10(-8) and 10(-6) M GRF. Secreted GH was measured at 4 hr after treatment. In EXP I, before injection, serum GH concentrations were similar. Serum GH concentrations increased (P < 0.01) after injection of 10 micrograms (21 +/- 1 ng/ml), 50 micrograms (9 +/- 1 ng/ml), and 100 micrograms (13 +/- 1 ng/ml) of leptin compared with S (1 +/- 2 ng/ml) treated pigs. The GH response to leptin was greater (P < 0.001) in 10 micrograms than 50 or 100 micrograms leptin-treated pigs. By 20 hr the 10, 50, and 100 micrograms doses of leptin reduced feed intake by 53% (P < 0.08), 76%, and 90% (P < 0.05), respectively, compared with S pigs. Serum IGF-1, insulin, T4, glucose, and free fatty acids were unaffected by leptin treatment. In EXP II, relative to control (31 +/- 2 ng/well), 10(-10), 10(-8), and 10(-6) M GRF increased (P < 0.01) GH secretion by 131%, 156%, and 170%, respectively. Only 10(-6) M and 10(-7) M leptin increased (P < 0.01) GH secretion. Addition of 10(-11) and 10(-9) M leptin in combination with 10(-6) M GRF or 10(-11) M leptin in combination with 10(-8) M GRF-suppressed (P < 0.05) GH secretion. These results indicate that leptin modulates GH secretion and, as shown in other species, leptin suppressed feed intake in the pig.

Increased pituitary growth hormone-releasing factor (GRF) receptor messenger ribonucleic acid expression in food-deprived rats

In prolonged food-deprived rats, growth hormone (GH) secretion in response to exogenous GH-releasing factor (GRF) is enhanced both in vivo and in vitro. We tested the hypothesis that, in fasted rats, GRF receptors in the pituitary may be up-regulated. The expression of mRNA for the GRF receptor in the pituitary and GRF binding to the pituitary membrane were examined in adult male Wistar rats deprived of food for 72 h. The level of GRF receptor mRNA in the pituitary was significantly increased after 48 h food deprivation and was more than 3 times the level in control rats after 72 h food deprivation. GRF binding to the pituitary was significantly increased after 72 h food deprivation. The results of the present study provide evidence regarding changes in the GH axis in fasted rats, involving increased pituitary responsiveness to GRF and an increase in the pituitary membrane GRF receptor concentration.

Effects of hyper- and hypoglycemia on blood growth hormone level in free-feeding rats with anterolateral deafferentation of the medial basal hypothalamus

In rats with anterolateral deafferentation of the medial basal hypothalamus, the growth hormone (GH) level in the blood showed irregular and small fluctuations instead of the usual high bursts and low trough level, and the baseline GH level was higher than that in sham-operated rats. Continuous infusion of a glucose solution to operated rats increased the baseline level, GH pulse and pulse amplitude. I.v. bolus injection of the glucose solution resulted in a significant but transient increase in GH level. Insulin-induced hypoglycemia decreased the blood GH level in operated rats more effectively than in sham-operated ones and that was prevented by simultaneous infusion of glucose. Since SS influence on GH secretion had been largely eliminated in rats with anterolateral deafferentation of the medial basal hypothalamus, it is highly unlikely that the effects of hyperglycemia or hypoglycemia on GH secretion were the consequence of altered SS secretion.

Central glucoprivation evoked by administration of 2-deoxy-D-glucose induces expression of the c-fos gene in a subpopulation of neuropeptide Y neurons in the rat hypothalamus

Central glucoprivation evoked by the intracerebroventricular administration of 2-deoxy-D-glucose (2DG) induces eating and suppresses growth hormone (GH) secretion in rats. To elucidate the hypothalamic mechanism of these phenomena, the induction of c-fos gene expression was examined by in situ hybridization using rats with centrally administered 2DG. Autoradiography on X-ray film showed that c-fos gene expression was transiently induced in discrete hypothalamic regions; namely the paraventricular nucleus, arcuate nucleus (ARC), the surrounding regions of the third ventricle dorsal to the ARC, and the periventricular nucleus (PeV). The time course of the expression was different in these nuclei. Double-label in situ hybridization for c-fos mRNA and neuropeptide Y (NPY) or somatostatin mRNAs revealed that 20% of the NPY neurons in the ARC expressed the c-fos gene, while a small population of somatostatin neurons (6.1% in the ARC and 2.6% in the PeV) expressed the c-fos gene following 2DG administration. Since NPY is an orexigenic neuropeptide and has an inhibitory effect on GH secretion, the data suggest that the activation of a subpopulation of NPY neurons in the ARC contributes, in part, to the increased food intake and suppression of GH secretion after central glucoprivation evoked by 2DG.

Caloric intake stimulates growth hormone secretion in food-deprived rats with anterolateral deafferentation of the medial basal hypothalamus or administered antiserum to somatostatin

In rats, food deprivation inhibits episodic growth hormone (GH) secretion. On the basis of previous studies, we hypothesized that during a recovery from prolonged fasting, caloric intake stimulates the release of GH-releasing factor (GRF) and this process does not depend on the specific macronutrients in the meal, while protein in the meal acts to restore characteristic ultradian rhythmicity of GH secretion. To test this hypothesis, the effect of caloric intake on GH secretion was examined in fasted adult male Wistar rats devoid of somatostatin (SS) influence on GH secretion either by anterolateral deafferentation (ALC) of the medial basal hypothalamus (MBH) or administration of anti-SS goat serum (ASS). Rats were provided with an indwelling right atrial cannula and were deprived of food for 72 h. ALC was performed 2 weeks prior to the study. ASS was given i.v. 8 h and 7 h prior to refeeding, respectively. Serial blood specimens were collected every 10 min. In rats with ALC (ALC rats) or rats given ASS (ASS rats), the blood GH level revealed irregularly occurring small fluctuations, instead of the usual high bursts and low trough level. The baseline GH level and the mean GH level of fasted ALC rats or fasted ASS rats were significantly lower than those of fed ALC rats or fed ASS rats. Feeding the isocaloric mixed meal, the protein meal or the protein-deficient meal increased the GH pulse frequency, the pulse amplitude, the baseline GH level and the mean GH level in 72-h fasted ALC rats. These changes in GH secretory pattern persisted during the period of observation and were independent of the type of meal ingested. Following feeding the mixed meal, similar changes in the GH secretory pattern demonstrated in 72-h fasted ALC rats were also observed in 72-h fasted ASS rats, suggesting that the stimulation of GH secretion following caloric intake is not limited to ALC rats. Since the influence of SS on GH secretion has been largely eliminated in ALC or ASS rats, it is highly unlikely that the augmentation of GH secretion following feeding after prolonged food deprivation was the consequence of inhibition of SS secretion. Although GRF measurement was not performed, it is conceivable that the signal of caloric intake is conveyed to the MBH and acts to stimulate GRF release.

Neuropeptide Y: a central regulator of energy homeostasis

Neuropeptide Y (NPY) is a 36 amino acid peptide belonging to the pancreatic polypeptide family of neuroendocrine hormones. It is the most abundant peptide yet discovered in the mammalian brain and is widely expressed by neurons in the central and peripheral nervous systems as well as adrenal medullary cells. Recently, a large number of studies have focussed on the potential roles played by NPY within the hypothalamus and pituitary with respect to the control of food intake and energy homeostasis. It is now clear that NPY is a potent stimulator of food intake in models of hyperphagia, that hypothalamic NPY also regulates sympathetic neural activity and it appears that NPY may also influence the glucocorticoid, growth hormone and thyroid hormone axes. Taken together, current data suggest that hypothalamic and pituitary NPY-expressing cells represent an important and critical site of integration of peripheral hormonal signals with regulation of energy homeostasis.