Vasoactive intestinal peptide is a hypothalamic prolactin-releasing neuropeptide in the turkey (Meleagris gallopavo)

The hypothesis that vasoactive intestinal peptide (VIP) functions as a hypothalamic prolactin (PRL)-releasing peptide in the turkey was tested by determining the effects of hypothalamic VIP immunoneutralization and pituitary VIP receptor blockade on hypothalamic extract (HE)-induced PRL secretion from dispersed anterior pituitaries. Incubation of cells with porcine VIP (pVIP; 0.5 or 10 nM) significantly stimulated PRL secretion. This effect was inhibited in a dose-related manner by 1-hr preincubation of pVIP with a VIP antisera (A/S; 1:500-1:50,000). Likewise, HE (0.3 equivalent)-stimulated PRL secretion was inhibited by preincubation with VIP A/S (P less than 0.0001). A 96-98% reduction in PRL secretion was obtained from cells cultured with HE, that was previously incubated with 1/500 dilution of antiserum. Pretreatment of pituitary cells for 15 min with [4Cl-D-Phe6,Leu17] VIP, a VIP receptor antagonist (10(-5) M), significantly depressed the PRL response to 0.5 nM VIP (9.9 +/- 0.5 micrograms/500,000 cells vs 4.9 +/- 0.1 micrograms/500,000 cells; 22.4 +/- 0.9 micrograms/500,000 cells vs 14.7 +/- 0.4 micrograms/500,000 cells) or 0.3 eq HE (8.8 +/- 0.6 micrograms/500,000 cells vs 5.2 +/- 0.2 micrograms/500,000 cells; 15.3 +/- 0.3 micrograms/500,000 cells vs 8.2 +/- 0.2 micrograms/500,000 cells). These results suggest that hypothalamic stimulation of PRL secretion appears to be mediated by receptors specific for VIP and that VIP is an endogenous hypothalamic PRL-releasing peptide in the turkey.