CONTROL OF LUTEINIZING-HORMONE AND PROLACTIN SECRETION IN BIRDS

In birds, the secretion of luteinizing hormone (LH) and prolactin is c ontrolled by the releasing hormones, gonadotrophin releasing hormone-I (GnRH-I) and vasoactive intestinal polypeptide (VIP), respectively. T he secretion of LH is depressed and the secretion of prolactin is at i ts highest, during incubation/brooding young and during the developmen t of reproductive photorefractoriness. In incubating domestic chickens , decreased LH and increased prolactin secretion are correlated with d ecreased hypothalamic GnRH-I mRNA and increased hypothalamic VIP mRNA. Increased plasma prolactin contributes to the suppression of LH secre tion in incubating poultry, possibly acting at the levels of the hypot halamus and the anterior pituitary gland. During the development of ph otorefractoriness, decreased plasma LH and increased plasma prolactin are correlated with decreased hypothalamic GnRH-I and increased hypoth alamic VIP. In birds exposed to seasonal changes in daylength, the sea sonally 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 photorefractorine ss, high concentrations of plasma prolactin play a role in ensuring th e rapid regression of the gonads but are not essential for the mainten ance of the condition. (C) 1998 Elsevier Science Inc. All rights reser ved.