The reproductive 'eye'? Immunocytochemical studies on the role of encephalic photoreceptors in the regulation of seasonal reproduction in birds

This dissertation examines the roles of encephalic photoreceptors (EPs) in regulating seasonal reproductions in birds. It is generally accepted that seasonal reproduction in birds is mediated through EPs. However, it is still unknown in birds about the mechanisms of EPs mediated photoperiodic time measurement. In this dissertation, firstly, I review the classic literature on EPs for circadian entrainment and photoperiodic time measurement as well as recent progress on the localization of the EPs and their possible connection with hypothalamic-pituitary-gonad axis. Secondly, I examine the distribution of the EPs immunocytochemically with the antibody, RET-P1 targeting the opsin protein, the photosensitive molecule within the photoreceptors in house sparrow, Passer domesticus and Gambel's white-crowned sparrow, Zonotrichia leucophlys gambelii. The locations of EPs were indicated in lateral septum, infundibulum and median eminence in both species, and in subseptal organ only in white-crowned sparrow. In order to test whether opsin synthesis in EPs display daily oscillation, I investigated the daily change of opsin and vasoactive intestinal peptide (VIP, perviously indicated expressed within EPs) immunoreactivity between morning (photoinducible phase) and midnight (non-photoinducible phase) in house sparrow held on short days. Thirdly, I examine the activities of EPs, gonadotropin-releasing hormone (GnRH) and VIP neurons among morning, midnight and 1 long day stimulated midnight in both photosensitive short day birds and photorefractory long day birds in Gambel's white-crowned sparrow. Finally, I examine the circadian involvement in EPs, GnRH and VIP under constant dim light in both photosensitive and photorefractory white-crowned sparrows. My research work suggests not only the multiple EPs mediated photic input pathways for regulating reproductive axis among avian species, but also the functions of EPs as both photoreceptor and pacemaker, which are involved in interpreting the photoperiodic signal. Among avian species, EPs may interact with GnRH and VIP system in different way to regulate seasonal reproduction, which may be responsible for different reproductive photoperiodism in birds.