EFFECT OF OVINE PROLACTIN ADMINISTRATION ON HYPOTHALAMIC VASOACTIVE-INTESTINAL-PEPTIDE (VIP), GONADOTROPIN-RELEASING HORMONE-I AND HORMONE-II CONTENT, AND ANTERIOR-PITUITARY VIP RECEPTORS IN LAYING TURKEY HENS

The effect of ovine prolactin (oPRL) administration to laying and incu bating turkey hens was studied. In experiment one, five treatment grou ps (n = 6) of laying hens received injections of vehicle or oPRL (4 mg /bird/day) for 2, 4, 8, or 14 days. Hypothalamic vasoactive intestinal peptide (VIP) content decreased to its lowest level (p < 0.05) by Day 2 of oPRL injection. Serum turkey PRL decreased by Day 8 of injection , reaching nadir by Day 14. Similar to the decline in hypothalamic VIP content, the maximal decline in the number of anterior pituitary VIP binding sites was achieved by Day 2 of oPRL administration. Hypothalam ic GnRH-1 content decreased after 2 days of oPRL injection and remaine d low through Day 14 of the experiment. GnRH-II levels declined with t ime, reaching significantly lower levels after 8 days of injections, a nd remained low through Day 14. Plasma LH levels also declined (p < 0. 05) after Day 14 of oPRL administration. In experiment 2, two groups ( n = 6) of incubating birds were used: controls receiving injections of vehicle only and birds receiving injections of oPRL (4 mg/bird/day) f or 10 days. Exogenous oPRL had no effect on hypothalamic VIP, GnRH-I o r II, anterior pituitary VIP binding sites, or plasma turkey PRL or LH . The findings suggest that in laying hens, PRL inhibits its own secre tion by acting on both the hypothalamus and the anterior pituitary; th is phenomenon does not occur in incubating birds. Furthermore, we prov ide evidence that PRL acts centrally to reduce LH levels by reducing G nRH levels in the hypothalamus.