Photoperiodic Termination of Hamster Refractoriness: Participation of the Pineal Gland

Hamsters exposed to short daylengths undergo testicular involution followed by recrudescence and refractoriness to the regressive effects of such photoperiods. Insensitivity of pineal target tissues to the antigonadal actions of melatonin plausibly accounts for this refractoriness. The present experiments indicate that insensitivity to melatonin persists in the absence of the pineal gland and confirm that long-day treatments which break refractoriness to short photoperiods also restore sensitivity to melatonin. The ability of long days to resensitize the neuroendocrine axis to melatonin was shown to require participation of the pineal gland. Thus the pineal mediates the effects of both long and short daylengths in phasing the seasonal reproductive cycle of the hamster.