PHYSIOLOGY OF HYPOTHALAMIC NEUROHORMONES: A novel hypothalamic neuropeptide with 38 residues was isolated from ovine hypothalamic tissues based on its activity of adenylate cyclase stimulation in rat pituitary cell culutres. The amino acid sequence was determined and the synthetic peptide was found to be as active as the native peptide in terms of adenylate cyclase stimulating activity (ACSA). The shortened form with 27 residues was subsequently isolated and found to be as active as the 38 residue peptide. These peptides were named PACAP38 (Pituiary Adenylate Cyclase Activating Polypeptide with 38 residues) and PACAP27, repectively. Both of these peptides possessed amidated C-terminus. Although these peptides were isolated based on their ACSA in the pituitary cells, their physiological role remains unknown. Therefore, we propose to investigate their regulatory role in the pituitary cell function using synthetic preparations of PACAPs. The immediate goal of the study is to determine the pituitary target cells for PACAP using histochemical and immunohistochemical method in rats. Then we will examine if PACAP meets the requirement for a physiological hypophysiotropic hormone. The requirement includes (i) presence of dense net-work of immunopositive nerve fibers in the external layer of the median eminence; (ii) direct action of the peptide on the pituitary cells and presence of specific high affinity binding site in these cells (iii) greater concentration of PACAP in the hypophysial portal blood than in the peripheral blood; (iv) clarification of the end point of the action of PACAP in the pituitary cells. If PACAP is a physiological regulator of the pituitary gland, a negative or positive feed-back mechanism may interplay between the pituitary and PACAP neurons in the hypothalamus. This problem will be investigated using immunohistochemistry, RIA for determination of peptide, and PACAP mRNA in the hypothalamus under various experimental conditions. The physiological actions of PACAP will also be examined by the immnunoneutralization of the action of endogenous PACAP.