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. Injection 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 1Ag (4.7-1175 pmol) Neuropeptide Y (NPY) was isolated from porcine brain by Tatemoto and co-workers (1, 2). It contains 36 amino acids and displays marked sequence homology to avian and bovine pancreatic polypeptide (APP and BPP). NPY-like immunoreactivity has been localized throughout the brain (3, 4, t, t), and especially high concentrations (5, 6) have been measured in the hypothalamus of the rat and human by RIA. NPY appears to be located in certain brainstem noradrenergic and adrenergic nuclei that project to the hypothalamus and also in intrinsic hypothalamic neurons (3, 4, t, t (225-250 g) were bilaterally ovariectomized while they were under ether anesthesia, 3-5 weeks before implantation of a 3V cannula or before decapitation for collection of anterior pituitary glands. The animals were housed in group cages under controlled conditions of temperature (23-25°C) and illumination (lights on 0500-1900) and were provided food and water ad lib.Jugular Cannulation. Twenty-four hours prior to 3V injection of NPY or saline (0.9% NaCl), a cannula was inserted into the right external jugular vein by using an established procedure (7). On the day of the experiment, the cannula was connected to a longer flexible tube (PE-50, Clay Adams) containing heparin and saline. This system allowed rapid collection of blood samples (0.6 ml) and replacement of blood volume with 0.9% NaCl without disturbing the animal. Rats were acclimated to these conditions for at least 1 hr prior to 3V injection of NPY.Ventricular Injections and Blood Sampling. A stainless steel cannula (23 gauge) was implanted into the third ventricle 7-10 days before the experiment (8). Various doses of synthetic porcine NPY (lots 003778 and 004219, Peninsula Laboratories, Belmont, CA) or an equal volume (2 ,u) of diluent (0.9% NaCl) was injected into the third ventricle of conscious, unrestrained animals after withdrawal of a preinjection blood sample (0.6 ml) through the jugular cannula. This blood sample was designated the 0-min sample and subsequent samples were obtained at various times after 3V injection. Samples were stored on ice and later spun at low speed at 4°C, the supernatant was removed, and plasma was divided into aliquots immediately or stored frozen for RIA to determine the concentrations of luteinizing hormone (LH) and growth hormone (GH) as well as follicle-stimulating hormone (FSH) in one experiment.Anterior Pituitary Cell Perif...