Control of luteinizing hormone and prolactin secretion in birds.

In birds, the secretion of luteinizing hormone (LH) and prolactin is controlled by the releasing hormones, gonadotrophin releasing hormone-I (GnRH-I) and vasoactive intestinal polypeptide (VIP), respectively. The secretion of LH is depressed and the secretion of prolactin is at its highest, during incubation/brooding young and during the development of reproductive photorefractoriness. In incubating domestic chickens, decreased LH and increased prolactin secretion are correlated with decreased hypothalamic GnRH-I mRNA and increased hypothalamic VIP mRNA. Increased plasma prolactin contributes to the suppression of LH secretion in incubating poultry, possibly acting at the levels of the hypothalamus and the anterior pituitary gland. During the development of photorefractoriness, decreased plasma LH and increased plasma prolactin are correlated with decreased hypothalamic GnRH-I and increased hypothalamic VIP. In birds exposed to seasonal changes in daylength, the seasonally maximal concentrations of plasma prolactin associated with the development of photorefractoriness can be explained, in part, by the saturation daylength for photoinduced prolactin release being reached in late spring/mid summer. During the development of photorefractoriness, high concentrations of plasma prolactin play a role in ensuring the rapid regression of the gonads but are not essential for the maintenance of the condition.