Neuropeptide Y administered chronically into the lateral ventricle profoundly inhibits both the gonadotropic and the somatotropic axis in intact adult female rats.
Neuropeptide Y (NPY) is known to be involved in the central regulation of appetite, sexual behavior, and reproductive functions. Whereas central administration of NPY strongly stimulates feeding in satiated animals, diet restriction produces overexpression of NPY in the arcuate and paraventricular nuclei that might reflect behavioral adaptations to shortage of food. Previous studies indicated that central administration of NPY resulted in controversial actions on LH secretion, either stimulatory or inhibitory. In order to analyze the chronic effect on pituitary function of centrally administered NPY, stainless steel cannulae were implanted in the right lateral ventricles of intact 45-day-old Sprague-Dawley female rats. Ten days later, Alzet osmotic minipumps filled with saline or different concentrations of NPY adjusted to deliver 3, 6, 12, or 18 micrograms/day were connected to the intracerebroventricular (icv) cannulae, implanted sc dorsally, and the effects of these treatments evaluated after 7 days. Chronic icv infusion of NPY produced the expected dose-related increase in food intake [25.3 +/- 0.8 g/day (basal) to 47.9 +/- 4.3 g/day (highest NPY dose)] and body wt gain (3.7 +/- 0.4-11.5 +/- 1.4 g/day). Basal insulinemia was highly correlated to the increase in food intake. This orexigenic action of NPY was accompanied by a drastic dose-related decrease in pituitary wt (14.0 +/- 0.5-8.3 +/- 0.3 mg), pituitary concentration of GnRH receptors, a known marker of the activity of the hypothalamo-pituitary gonadal axis (15.2 +/- 1.7-5.2 +/- 0.5 fmol/mg), and ovarian wt (84.0 +/- 4.2-49 +/- 6.7 mg). Ovulation was impaired in NPY-treated animals as seen by daily inspection of vaginal smears. A sharp dose-dependent decrease in plasma levels of insulin-like growth factor I was also observed [934 +/- 64 ng/ml (basal) to 385 +/- 26 ng/ml (highest NPY dose)], probably secondary to a decrease in GH secretion. Whereas these data confirm the central action of NPY to stimulate appetite in satiated animals, they provide the first demonstration that chronic icv administration of NPY unequivocally inhibits gonadotropin secretion and sexual function in intact female rats. These data also confirm that NPY can suppress GH secretion and other anabolic hormones. In conclusion, these results may indicate a physiological role of NPY as an integrator of different adaptive behaviors in periods of unfavorable metabolic conditions such as diet restriction, extending its action to inhibition of sexual functions and anabolic processes.