INVOLVEMENT OF THE Y-1 RECEPTOR SUBTYPE IN THE REGULATION OF LUTEINIZING-HORMONE SECRETION BY NEUROPEPTIDE-Y IN RATS

The present experiments were designed to investigate structure-function relationships, and identify the receptor subtype and postreceptor cellular mechanisms that mediate the ovarian hormone-dependent, excitatory, and inhibitory effects of neuropeptide Y (NPY) on LH release in female rats. Intracerebroventricular administration of NPY decreased plasma concentrations of LH in ovariectomized, hormonally untreated rats but stimulated LH release in ovariectomized rats pretreated with estradiol benzoate and progesterone. A similar dual response was also obtained after administration of NPY2-36. However, deletion of additional amino acids at the N terminus, as in NPY5-36, NPY11-36, NPY16-36, and NPY25-36, rendered the peptides inactive. An N-terminal fragment, NPY1-24-amide, and a discontinuous NPY analog, NPY1-4-epsilon-amino-caproic acid-25-36, similarly failed to influence LH release. The analog [Leu31,Pro34]NPY, a preferential agonist at the Y-1 NPY receptor subtype, also elicited the dual LH responses, but the preferential Y-2 receptor agonist, NPY13-36, was completely inactive. Further, only the peptides that stimulated LH release in vivo, i.e. NPY, NPY2-36, and [Leu31,Pro34]NPY, also stimulated the release of LHRH from median eminence fragments of steroid-primed rats in vitro, and the excitatory effect of [Leu31,Pro34] NPY was blocked by a noncompetitive NPY receptor antagonist, D-myo-inositol-1,2,6-trisphosphate. With the exception of NPY1-24-amide, all of the NPY fragments tested bound specifically to NPY binding sites in hypothalamic membrane preparations, but the highest binding affinities were found for the peptides that evoked biological responses in the in vivo and in vitro tests. Further analysis of the mode of action of NPY showed that the stimulation of LHRH release in vitro was unaffected by omission of Ca2+ from the incubation medium, but was prevented by two antagonists of intracellular Ca2+ mobilization, 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester and ryanodine. Inhibition of prostaglandin synthesis with indomethacin blocked the stimulatory effect of the alpha(1)-adrenergic agonist methoxamine on LHRH release, but not the increase produced by NPY. Cumulatively, these studies show that 1) both the inhibitory and stimulatory effects of NPY on LH secretion are exerted at the Y-1 NPY receptor subtype, which requires nearly the full NPY sequence for optimal activation; 2) the lack of effect of smaller fragments of NPY may be due to their decreased affinity for the NPY receptor; and 3) the excitatory LHRH response occurring subsequent to activation of the Y-1 receptor is mediated by mechanisms that mobilize Ca2+ from intracellular sources, but unlike the response to alpha(1)-receptor stimulation, does not involve synthesis of prostaglandins.