Effects of Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) and Vasoactive Intestinal Polypeptide (VIP) on Prolactin, Luteinizing Hormone and Growth Hormone Secretion in the Ewe

This study was undertaken to investigate the roles of PACAP and VIP in the control of pituitary hormone secretion in the ewe. The first experiment was designed to identify any direct effects at the level of the pituitary and was conducted during the luteal phase of a prostaglandin‐synchronized oestrous cycle. PACAP (0.008, 0.04, 0.2 and 1.0 nmol/min) or VIP (0.06, 0.2, 0.6 and 1.8 nmol/min) was infused into the carotid artery over a 10 min period. Blood samples were taken before and after the infusions so that plasma PRL, LH and GH concentrations could be measured. Blood pressure was also monitored to determine if the doses used were biologically active. In no case was an effect on hormone secretion observed. In contrast, the highest dose of each peptide induced an increase in heart rate to almost three‐fold the resting value. Although both peptides are active in vivo, this result suggests that neither peptide has a direct effect on hormone release from the pituitary of prostaglandin‐synchronized ewes. In a second experiment, we investigated whether the peptides had central effects on hormone secretion. Intracerebroventricular (ICV) injection of PACAP or VIP at the dose 10nmol was tested in ovariectomized ewes. After injection, PACAP suppressed PRL and GH secretion so that plasma hormone concentrations from 1–3 h after injection were significantly different from the control (P