Effect and site of action of hypothalamic neuropeptides on prolactin release in sheep

In an attempt to identify a physiological prolactin-releasing factor in the sheep, ovariectomized ewes were given intracarotid injections (10(-8)-10(-7) mol/animal) of thyrotropin-releasing hormone (TRH), vasoactive intestinal polypeptide (VIP), peptide histidine-isoleucine amide (PHI), oxytocin (OT), arginine vasopressin (AVP), substance P (SP), bombesin (BB), neurotensin (NT) and neuropeptide Y (NPY). Administration of TRH, AVP, NT and OT resulted in immediate and significant increases in plasma prolactin concentrations, the greatest stimulatory effect being obtained after TRH; other peptides had no effect in ovariectomized hypothalamo-pituitary intact ewes. AVP, NT and OT failed to release prolactin in ovariectomized ewes. These results suggest that (1) AVP, NT and OT may act via the hypothalamus to regulate prolactin secretion in hypothalamo-pituitary intact ewes; (2) VIP, PHI, SP, BB and NPY appear to have no direct roles at the pituitary level to control prolactin secretion in sheep, and (3) TRH stimulates prolactin secretion in ovariectomized ewes by a direct pituitary action.