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Neuropeptide Y: a novel neuroendocrine peptide in the control of pituitary hormone secretion, and its relation to luteinizing hormone.

Evidence that establishes neuropeptide Y (NPY) as an important neuromessenger in the regulation of anterior pituitary hormone secretion is reviewed. In particular, NPY plays a critical role in stimulating the episodic, basal pattern of luteinizing hormone (LH) release, as well as the preovulatory surge of LH release in several species. The stimulatory effect of NPY on LH secretion is dependent upon the presence of gonadal hormones and involves amplification of the response of other interacting stimulatory signals. NPY acts at the level of the median eminence to excite the release of LH-releasing hormone (LHRH) via a mechanism that leads to the mobilization of intracellular Ca2+. These stimulatory LHRH responses are mediated by Y1NPY receptors. Moreover, NPY activates postsynaptic messenger pathways that complement and reinforce those affected by norepinephrine, which is another major neuroregulator of LHRH secretion and which is released as a cotransmitter with NPY in the median eminence. Additionally, NPY is released into the hypophyseal portal blood for transportation to the anterior pituitary where it enhances the release of LH in response to LHRH. This facilitatory, modulating effect at the pituitary level involves an allosteric increase in LHRH binding to its receptor leading to augmented influx of Ca2+ from the extracellular space. There is evidence that gonadal steroids regulate NPY neurosecretion in a site-specific manner, and that alterations in NPY secretion may occur in part via a direct action of the steroids on NPY neurons in the brainstem and hypothalamus and in part through an indirect effect involving removal of the inhibitory influence of endogenous opioid peptides. These findings are integrated into an overall hypothesis for induction of the preovulatory LH surge on proestrus requiring an interplay between NPY and other neuronal networks. In aged male rats, due to the inability of hypothalamic NPY neurons to respond appropriately to the trophic effects of androgens, NPY neurosecretion is diminished. Further, a review of the literature reveals that NPY may modulate the secretion of other pituitary hormones through a similar combination of hypothalamic and pituitary actions.