Role of vasoactive intestinal peptide in the control of prolactin-induced turkey incubation behavior. I. Acute infusion of vasoactive intestinal peptide.

Vasoactive intestinal peptide (VIP) stimulates prolactin (PRL) secretion. Ovine PRL induces incubation behavior in avian species. This study was designed to determine whether VIP can elevate plasma PRL for up to 3 h. Saline or porcine VIP (pVIP; 30, 60, or 150 ng/min) was infused into the median eminence of laying turkeys for 1 h. The 60- and 150-ng doses of pVIP increased plasma PRL (p < 0.01), whereas the 30-ng dose was insignificant. Pituitary PRL content decreased in pVIP-treated turkeys. Two-hour infusion of 60 or 150 ng chicken VIP (cVIP)/min produced similar elevations of plasma PRL (p < 0.001), which declined within 80 min. Both treatments induced insignificant increases in pituitary PRL mRNA. Saline or cVIP (30, 60, or 60 [pulsed] ng/min) was infused into the median eminence for 3 h. Sixty ng cVIP/min induced the largest PRL release (p < 0.05). The pulsatile and low-cVIP treatments resulted in release of a significant amount of PRL in comparison to the saline treatment (p < 0.01). All cVIP treatments resulted in decreased pituitary PRL content (p < 0.05). The 60-ng dose increased PRL mRNA (p < 0.1). This study shows that 60 ng VIP/min causes the maximum PRL release in laying turkeys. However, pituitary PRL content is depleted and PRL synthesis cannot maintain PRL secretion at high levels.