Vasoactive intestinal peptide concentrations in turkey hypophysial portal blood differ across the reproductive cycle

Hypophysial portal blood was collected for the first time in an avian species using a dorsal approach through the third ventricle and median eminence of the turkey. This was done to test for the presence of the prolactin (PRL)-releasing factor vasoactive intestinal peptide (VIP) in the portal blood and to determine whether VIP concentrations there varied with and corresponded to plasma PRL levels across the reproductive cycle. VIP concentrations in hypophysial portal blood were 2.5- to 13.5-fold greater than in the general circulation. VIP concentrations were lowest in portal blood of nonphotostimulated, reproductively inactive hens (231.8 +/- 26.4 pg/ml) and highest in incubating hens (1108.1 +/- 363.7 pg/ml), while laying, and photorefractory hens were intermediate at 372.5 +/- 95.6 and 715.3 +/- 338.5 pg/ml, respectively. These differences in concentration of VIP in portal blood mirrored those of PRL in the general circulation and support other evidence that VIP is the avian PRL-releasing factor.