An inverse relationship often exists between prolactin (Prl) and LH in avian species. Our study was designed to investigate the relationship between hypothalamic and posterior pituitary vasoactive intestinal peptide (VIP)-an endogenous Prl-releasing peptide-and chicken GnRH-I and GnRH-II. Hypothalamic VIP content was increased after photostimulation, reaching its highest levels in incubating and photorefractory birds. The highest hypothalamic GnRH-I content was in laying hens followed by that in photostimulated and incubating birds. The lowest levels were in the nonphotostimulated birds. Hypothalamic GnRH-II increased after photostimulation, then fell to nonphotostimulated levels during incubation and photorefractoriness. Posterior pituitary VIP content was elevated in response to photostimulation, reaching a peak value in the laying and incubating birds, then declining in the photorefractory hens. Posterior pituitary GnRH-I and GnRH-II content peaked in the incubating birds. Ovariectomy caused a significant reduction in hypothalamic GnRH-I and GnRH-II with no effect on VIP; no changes were found in the posterior pituitary due to ovariectomy. Reducing day length caused a significant decrease in the hypothalamic and the posterior pituitary content of VIP and GnRH-I, and GnRH-II. Ovine Prl (oPrl) administration to laying hens reduced the hypothalamic VIP and GnRH-I and GnRh-II content. Posterior pituitary GnRH-I content was also reduced. Although GnRH-II levels were reduced by Day 4 of injections, they rose to peak levels by Day 14 of oPrl administration. Posterior pituitary VIP content was not altered by oPrl.
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