Chronic blockade of the melanocortin 4 receptor subtype leads to obesity independently of neuropeptide Y action, with no adverse effects on the gonadotropic and somatotropic axes.

Neuropeptide Y (NPY) is a powerful orexigenic factor, and alphaMSH is a melanocortin (MC) peptide that induces satiety by activating the MC4 receptor subtype. Genetic models with disruption of MC4 receptor signaling are associated with obesity. In the present study, a 7-day intracerebroventricular infusion to male rats of either the MC receptor antagonist SHU9119 or porcine NPY (10 nmol/day) was shown to strongly stimulate food and water intake and to markedly increase fat pad mass. Very high plasma leptin levels were found in NPY-treated rats (27.1 +/- 1.8 ng/ml compared with 9.9 +/- 0.9 ng/ml in SHU9119-treated animals and 2.1 +/- 0.2 ng/ml in controls). As expected, NPY infusion induced hypogonadism, characterized by an impressive decrease in seminal vesicle and prostate weights. No such effects were seen with the SHU9119 infusion. Similarly, whereas the somatotropic axis of NPY-treated rats was fully inhibited, this axis was normally activated in the obese SHU9119-treated rats. Chronic infusion of SHU9119 strikingly reduced hypothalamic gene expression for NPY (65.2 +/- 3.6% of controls), whereas gene expression for POMC was increased (170 +/- 19%). NPY infusion decreased hypothalamic gene expression for both POMC and NPY (70 +/- 9% and 75.4 +/- 9.5%, respectively). In summary, blockade of the MC4 receptor subtype by SHU9119 was able to generate an obesity syndrome with no apparent side-effects on the reproductive and somatotropic axes. In this situation, it is unlikely that hyperphagia was driven by increased NPY release, because hypothalamic NPY gene expression was markedly reduced, suggesting that hyperphagia mainly resulted from loss of the satiety signal driven by MC peptides. NPY infusion produced hypogonadism and hyposomatotropism in the face of markedly elevated plasma leptin levels and an important reduction in hypothalamic POMC synthesis. In this situation NPY probably acted both by exacerbating food intake through Y receptors and by reducing the satiety signal driven by MC peptides.

Evidence that the inhibition of luteinizing hormone secretion exerted by central administration of neuropeptide Y (NPY) in the rat is predominantly mediated by the NPY-Y5 receptor subtype.

A number of studies have indicated that neuropeptide Y (NPY) is a central regulator of the gonadotropic axis, and the Y1 receptor was initially suggested to be implicated. As at least five different NPY receptor subtypes have now been characterized, the aim of the present study was to reinvestigate the pharmacological profile of the receptor(s) mediating the inhibitory action of NPY on LH secretion by using a panel of NPY analogs with different selectivity toward the five NPY receptor subtypes. When given intracerebroventricularly (icv) to castrated rats, a bolus injection of native NPY (0.7-2.3 nmol) dose-dependently decreased plasma LH. Peptide YY (PYY; 2.3 nmol) was as potent as NPY, suggesting that the Y3 receptor is not implicated. Confirming previous data, the mixed Y1, Y4, and Y5 agonist [Leu31,Pro34]NPY (0.7-2.3 nmol) inhibited LH release with potency and efficacy equal to those of NPY. Neither the selective Y2 agonist C2-NPY (2.3 nmol) nor the selective Y4 agonist rat pancreatic polypeptide affected plasma LH, excluding Y2 and Y4 subtypes for the action of NPY on LH secretion. The mixed Y4-Y5 agonist human pancreatic polypeptide (0.7-7 nmol) as well as the mixed Y2-Y5 agonist PYY3-36 (0.7-7 nmol) that displayed very low affinity for the Y1 receptor, thus practically representing selective Y5 agonists in this system, decreased plasma LH with potency and efficacy similar to those of NPY, indicating that the Y5 receptor is mainly involved in this inhibitory action of NPY on LH secretion. [D-Trp32]NPY, a selective, but weak, Y5 agonist, also inhibited plasma LH at a dose of 7 nmol. Furthermore, the inhibitory action of NPY (0.7 nmol) on LH secretion could be fully prevented, in a dose-dependent manner (6-100 microg, icv), by a nonpeptidic Y5 receptor antagonist. This antagonist (60 microg, icv) also inhibited the stimulatory action of NPY (0.7 nmol) on food intake. The selectivity of PYY3-36, human PP, [D-Trp32]NPY, and the Y5 antagonist for the Y5 receptor subtype was further confirmed by their ability to inhibit the specific [125I][Leu31,Pro34]PYY binding to rat brain membrane homogenates in the presence of the Y1 receptor antagonist BIBP3226, a binding assay system that was described as being highly specific for Y5-like receptors. With the exception of [D-Trp32]NPY, all analogs able to inhibit LH secretion were also able to stimulate food intake. Taken together, these results indicate that the Y5 receptor is involved in the negative control by NPY of the gonadotropic axis.

Chronic administration of neuropeptide Y into the lateral ventricle of C57BL/6J male mice produces an obesity syndrome including hyperphagia, hyperleptinemia, insulin resistance, and hypogonadism.

Neuropeptide Y (NPY) is involved in the central regulation of appetite, sexual behavior, and reproductive function. We have previously shown that chronic infusion of NPY into the lateral ventricle of normal rats produced an obesity syndrome characterized by hyperphagia, hyperinsulinism and collapse of reproductive function. We further demonstrated that acute inhibition of LH secretion in castrated rats was preferentially mediated by the NPY receptor subtype 5 (Y(5)). In the present study, the effects of chronic, central infusion of NPY, or the mixed Y2-Y5 agonist PYY(3-36), were evaluated both in normal male C57BL/6J mice and Sprague-Dawley rats. After a 7-day infusion to male mice, both NPY and PYY(3-36) at 5 nmol per day, induced marked hyperphagia leading to significant increases in body and fat pad weights. Furthermore, both compounds markedly reduced several markers of the reproductive axis. In the rat study, PYY(3-36) was more active than NPY to inhibit the pituitary-testicular axis, confirming the importance of the Y5 subtype for such effects. In the mouse, chronic NPY infusion induced a sustained increase in corticosterone and insulin secretion. Plasma leptin levels were also markedly increased possibly explaining the observed reduction in gene expression for hypothalamic NPY. Gene expression for hypothalamic POMC was reduced in the NPY- or PYY(3-36)-infused mice, suggesting that NPY exacerbated food intake by both acting through its own receptor(s), and reducing the satiety signal driven by the POMC-derived alpha-MSH. The present study in the mouse suggests in analogy with available rat data, that constant exposure to elevated NPY in the hypothalamic area unabatedly enhances food intake leading to an obesity syndrome including increased adiposity, insulin resistance, hypercorticism, and hypogonadism, reminiscent of the phenotype of the ob/ob mouse, that displays elevated hypothalamic NPY secondary to lack of leptin negative feedback action.

The melanocortin agonist Melanotan-II reduces the orexigenic and adipogenic effects of neuropeptide Y (NPY) but does not affect the NPY-driven suppressive effects on the gonadotropic and somatotropic axes in the male rat.

Neuropeptide Y (NPY) is a strong orexigenic neurotransmitter also known to modulate several neuroendocrine axes. alpha-Melanocyte-stimulating hormone (MSH) is an essential anorectic neuropeptide, acting on hypothalamic MC3/4 receptor subtypes. When given as an intracerebroventricular bolus injection, Melanotan-II (MT-II), a non selective MC receptor agonist, inhibits feeding, suppresses the NPY orexigenic action, and reduces basal insulinaemia. We evaluated the effects of a 7-day central infusion of MT-II (15 nmol/day) given either alone or in association with NPY (5 nmol/day) in male Sprague-Dawley rats. MT-II produced almost full anorexia for 1-2 days but then feeding gradually returned to normal despite continued MT-II infusion. When coinfused with NPY, MT-II also produced the same initial anorectic episode but then maintained feeding to upper normal levels, thus cancelling the hyperphagia driven by NPY. Whereas NPY infusion produced a doubling of fat pad weight, MT-II reduced adiposity by a factor of two compared to pair-fed rats, and vastly curtailed the NPY-driven increase in fat pad weight. MT-II infusion also significantly curtailed the NPY-induced rise in insulin and leptin secretions. NPY infusion significantly inhibited hypothalamic pro-opiomelanocortin mRNA expression, most likely cancelling the alpha-MSH anorectic activity. As expected from previous studies, chronic NPY infusion strongly inhibited both the gonadotropic and somatotropic axes, and coinfusion of MT-II did not reverse these NPY-driven effects, in sharp contrast with that seen for the metabolic data. MT-II infusion alone had little effect on these axes. In conclusion, chronic MT-II infusion generated a severe but transient reduction in feeding, suggesting an escape phenomenon, and clearly reduced fat pad size. When coinfused with NPY, MT-II was able to cancel most of the NPY effects on feeding, but not those on the neuroendocrine axes. It appears therefore that, as expected, NPY and alpha-MSH closely interact in the control of feeding, whereas the neural pathways by which NPY affects growth and reproduction are distinct and not sensitive to MC peptide modulation.

Stimulation of the gonadotropic axis by the neuropeptide Y receptor Y1 antagonist/Y4 agonist 1229U91 in the male rat.

Neuropeptide Y (NPY) is a highly potent orexigenic substance that is also known to modulate gonadotropin secretion. Five receptor subtypes for NPY have been identified, and a potent antagonist for the receptor subtype 1 (Y1), 1229U91, also known as GW1229 or GR231118, has been described. Subsequently, 1229U91 was also shown to represent a highly potent agonist for the Y4 receptor subtype. Very unexpectedly, intracerebroventricular administration of 1229U91 elicited an intense, dose-dependent surge of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in intact male rats that lasted for 6 h. Such stimulation was absent when a potent gonadotropin-releasing hormone antagonist was administered systemically, suggesting that 1229U91 acts centrally to stimulate gonadotropin-releasing hormone release. 1229U91 administration had no effect on growth hormone, thyroid-stimulating hormone, and corticosterone secretions. In addition to 1229U91, four other parent dimer molecules described earlier produced a marked and sustained stimulation of LH when injected intracerebroventricularly that was proportional to their binding affinity for the Y4 receptor. Central administration of the specific Y1 antagonist BIBO3304 (20 microgram) had no effect on LH secretion, making it unlikely for 1229U91 to stimulate LH secretion by an antagonistic action on the Y1 receptor subtype, thus suggesting a Y4 receptor mediation. In conclusion, the 1229U91 molecule displays an interesting conformational epitope that is able to generate large LH surges, possibly by activating Y4 or Y4-like receptor subtypes or by acting on a NPY receptor unrelated target.