Photoperiodic regulation of the reproductive system in the male Syrian hamster (Mesocricetus auratus): Neuroendocrine and molecular aspects

This thesis investigated changes in neuroendocrine function associated with the transfer of male Syrian hamsters (Mesocricetus auratus) from long to short daylengths (LD 14:10 to LD 10:14). Such a shift initiates reproductive quiescence in this species. Four separate studies were conducted. The effect of testosterone and photoperiod on pulsatile luteinizing hormone (LH) secretion was first examined in acutely and chronically castrated male hamsters. Results indicated that testicular androgens are necessary for the initiation of reproductive regression but not for its maintenance. In other studies, hypothalamic luteinizing hormone releasing hormone (GnRH) and GnRH-associated peptide (GAP) biosynthesis and storage were analyzed in intact and castrated + testosterone-treated hamsters. Brains were processed with a free-floating procedure which allows immunocytochemistry and in-situ hybridization on alternate tissue sections. After four weeks of exposure to short daylengths, the number of GnRH- and GAP-immunoreactive cells increased significantly in the diagonal band of Broca and medial septum but not in other brain regions. This increase correlated with rises in steady state GnRH mRNA levels, as measured by in-situ hybridization and T...