S09-4 Molecular neuroendocrine mechanisms controlling photoperiodically regulated release of gonadotropin releasing hormone

Change in photoperiod is a key proximal factor timing seasonal breeding at higher latitudes. Photoperiodic information is transduced in the neuroendocrine system to regulate the secretion of neuropeptides, gonadotropin releasing hormoneI (GnRH-I) and a vasoactive intestinal polypeptide, which respectively regulate the release of luteinizing hormone (LH) and prolactin. The initiation of photoinduced LH secretion is closely associated with an increase in hypothalamic GnRH mRNA, suggesting that an early event in the photoperiodic reproductive response is an increase in GnRH gene transcription. In many photoperiodic species, seasonal breeding is terminated by the development of absolute or relative photorefractoriness. Absolute photorefractory birds become unresponsive to very long days, including continuous light, while relative photorefractory birds become unresponsive to spring-like day lengths but remain responsive to very long days. The development of both absolute and relative photorefractoriness is associated with decreased plasma LH and increased plasma prolactin. Photorefractoriness develops for prolactin secretion in absolute photorefractory birds but not in relative photorefractory quail, Coturnix. Prolactin treatment depresses GnRH-I and LH beta mRNAs. It is suggested that the development of absolute and relative photorefractoriness may be initiated by increased plasma prolactin, and that increased prolactin maintains relative, but not absolute photorefractoriness.