Hormonal modulation of pineal-mediated seasonal events: effects of thyroid or gonadal manipulations.

The ability of various manipulations of the endocrine system to affect pineal-mediated events was examined in the present studies. Male Syrian hamsters were analyzed for pineal-induced gonadal regression and depressions in serum thyroxine and testosterone levels after treatments which altered thyroid or gonadal function. Hamsters were thyroidectomized, received thyroxine implants (5 mg), or were thyroidectomized plus implanted with thyroxine. The animals were exposed to short photoperiods (10L:14D) for 9 wk, and plasma hormone levels and gonadal status were determined at the end of the experimental period. Likewise, hamsters were castrated, received testosterone implants (5 mg), or were castrated plus implanted with testosterone, and subsequently were exposed to short photoperiods for 9 wk. These animals' responses to photoperiod exposure were compared to animals which received identical hormonal treatments but remained in long photoperiod (14L:10D). All of the hamsters responded to the photoperiodic treatments equally, regardless of the hormonal treatment. The results of these studies indicate that experimentally induced alterations in plasma thyroxine or testosterone levels are unable to prevent or attenuate the ability of the pineal to elicit gonadal regression in response to short photoperiod exposure.