Evidence that vasoactive intestinal polypeptide is a physiological prolactin-releasing factor in the bantam hen

Vasoactive intestinal polypeptide (VIP)-like material was localised immunohistochemically in the hypothalamus of the bantam hen. Abundant immunoreactive VIP terminals were seen in the external layer of the median eminence and most immunoreactive VIP cell bodies were located in the basal hypothalamus. A few immunoreactive VIP cell bodies and many fibres were found in the preoptic hypothalamus. Intravenous injections of synthetic porcine VIP over a dose range of 12.5 to 100 micrograms kg-1 body wt resulted in dose-related increase in concentration of plasma prolactin in incubating bantams deprived of their nests for 24 hr. These doses of VIP did not stimulate the release of growth hormone. Studies in vitro showed that synthetic VIP directly stimulated prolactin release from the anterior pituitary gland. The glands from incubating bantams were more responsive to the prolactin-releasing effects of VIP than were the glands from laying birds. Studies in vitro showed that the amount of prolactin released in response to an iv injection of 50 micrograms kg-1 VIP was greater in incubating birds deprived of their nests for 24 hr than in laying hens. Prolactin release was not stimulated in ovariectomized hens after an injection of 50 micrograms kg-1 VIP unless the birds were first treated with oestrogen or oestrogen and progesterone. It was concluded that a VIP-like material in the bantam hypothalamus may be a physiological prolactin-releasing factor acting at least in part at the level of the anterior pituitary gland.(ABSTRACT TRUNCATED AT 250 WORDS)