Neuropeptide Y affects secretion of luteinizing hormone and growth hormone in ovariectomized rats

Neuropeptide Y (NPY) has recently been localized in the rat hypothalamus. We have evaluated the effects of NPY on hypothalamic and pituitary function by injecting NPY into the third ventricle in vivo and by examining its action on perifused pituitary cells in vitro. Injections of NPY into the third ventricle of conscious ovariectomized rats led to a dramatic and highly significant reduction in plasma luteinizing hormone (LH) relative to pretreatment levels in these animals or to those of controls injected with physiological saline. Significant inhibition was obtained with doses ranging from 0.02 to 5.0 micrograms (4.7-1175 pmol) of NPY. These inhibitory effects on LH release were dose dependent and lasted for at least 120 min after injection of 5.0 micrograms of NPY. Intraventricular injection of NPY also significantly decreased plasma growth hormone; however, the threshold dose was 2.0 micrograms (470 pmol), a dose 100-fold greater than the lowest dose that inhibited LH release. Plasma follicle-stimulating hormone was unaffected by injection of NPY. NPY (10(-6) and 10(-7) M) stimulated secretion of LH, growth hormone, and follicle-stimulating hormone from perifused anterior pituitary cells loaded in a Bio-Gel P-2 column. These results indicate that NPY acts on structures adjacent to the third ventricle to inhibit the secretion of LH and growth hormone but not follicle-stimulating hormone, whereas it can directly stimulate the secretion of all three hormones from the cells of the anterior pituitary in vitro. Since NPY has been found in the hypothalamus and median eminence, it is quite likely that it plays a physiologically significant role at both hypothalamic and pituitary sites: influencing secretion of pituitary hormones.