Immunocytochemical study of GnRH and GnRH-associated peptide in male Syrian hamsters as a function of photoperiod and gonadal alterations.

Hypothalamic luteinizing-hormone-releasing hormone (GnRH) and gonadotropin-releasing-hormone-associated peptide (GAP) biosynthesis and storage were estimated by immunocytochemistry in male golden hamsters maintained in different photoperiods. Intact or castrated male hamsters with subcutaneously inserted testosterone implants were exposed to long-day (14:10) or short-day photoperiods (10:14) for 4-8 weeks. Exposure to short photoperiod for 4 weeks, an interval characterized by a suppression of gonadotropin secretion but not gonadal regression, was associated with an increase in the number of GnRH- and GAP-immunoreactive cells in the diagonal band of Broca/medial septum. Furthermore, morphometric analysis revealed that these animals displayed significantly more GnRH but not GAP immunoreactivity in the median eminence as opposed to hamsters exposed to long-day photoperiods. In additional studies, gonadally regressed hamsters exposed to short day lengths for 8 weeks had equal numbers of GnRH cells as did the long-day controls. These patterns suggest that reproductive quiescence in golden hamsters is not the result of depletions of neuronal GnRH stores available for secretion.