Injection of neuropeptide Y into the third cerebroventricle differentially influences pituitary secretion of luteinizing hormone and growth hormone in ovariectomized cows

Hypothalamic neurons that control the luteinizing hormone (LH) and growth hormone (GH) axes are localized in regions that also express neuropeptide Y (NPY). Increased hypothalamic expression of NPY due to diet restriction has been associated with suppressed secretion of LH and enhanced secretion of GH in numerous species. However, these physiological relationships have not been described in cattle. Thus, two studies were conducted to characterize these relationships using ovariectomized (Experiment 1) or ovariectomized estrogen-implanted (Experiment 2) cows. In Experiment 1, four well-nourished, ovariectomized cows received third cerebroventricular (TCV) injections of 50 and 500 μg of NPY in a split-plot design. Venous blood was collected at 10-min intervals from −4 hr (pre-injection control period) to +4 hr (postinjection treatment period) relative to TCV injection. NPY suppressed (P ⩽ 0.04) tonic secretion of LH irrespective of dose and tended to stimulate (P ⩽ 0.10) an increase in tonic secretion of GH. In Experiment 2, six ovariectomized cows that were well nourished and implanted with estradiol received TCV injections of 0, 50, or 500 μg of NPY in a replicated 3 × 3 Latin Square. Both doses of NPY suppressed (P < 0.06) mean concentration of LH relative to the 0-μg dose. The 50-μg dose of NPY tended (P < 0.10) to increase the amplitude of GH pulses. In conclusion, TCV injection of NPY suppressed pituitary secretion of LH and simultaneously tended to increase pituitary secretion of GH.