Circadian organization and neural mediation of hamster reproductive rhythms

(1) Circadian rhythms mediate various cycles of the hamster's reproductive physiology and behavior. (2) Destruction of the suprachiasmatic nuclei interfered with the integration and/or generation of coherent circadian rhythms of wheel-running behavior and also abolished photoperiodic regulation of the testes, which remain large and functional after the lesion regardless of photoperiod or presence of the eyes. (3) The retinohypothalamic tract is the principal visual projection for the entrainment of circadian activity rhythms to the light: dark cycle and also appears necessary and sufficient for mediating the effects of photoperiod on the reproductive physiology of the male hamster. (4) The hamster estrous cycle free-ran under constant dim illumination with a period significantly different from 4 days; regression analyses of concurrently recorded free-running heat onsets and wheel-running behavior showed that the period of the estrous cycle was a multiple (× 4) of the circadian activity period. This implies a circadian organization of the estrous cycle and mediation of both activity and estrous behaviors by a common circadian system. Implications of these findings for contemporary views of sexual differentiation of gonadotrophin release are considered. (5) Estradiol influenced the phase angle of entrainment of wheel-running behavior by ovariectomized hamsters. The period of the free-running activity rhythm was significantly shortened in blind ovariectomized hamsters implanted with Silastic capsules containing estradiol benzoate. The functional significance of these hormone-mediated changes in rhythm periodicity is discussed. (6) Exposure of male hamsters to short daily photoperiods reduced the frequency of copulation. Castrated males maintained on short daily photoperiods were less responsive than those on long days to the activational effects of testosterone on copulation. The significance of this photoperiodically induced refractoriness to hormones, which in this species appears limited to the male, is discussed.