Evidence for refractoriness of the pituitary‐gonadal axis to the pineal gland in golden hamsters and its possible implications in annual reproductive rhythms

The testes of adult hamsters maintained in short daily photo‐periods (light:dark “LD” cycles of 1:23, in hours) undergo regression within ten weeks and spontaneous regeneration within about 30 weeks. Thereafter (at least up to 80 weeks), as long as these animals are kept in short photoperiods the gonads do not experience a second atrophic response. After the 30 week period of dark exposure, if the hamsters are moved into a long photoperiodic environment (LD 14:10) for either one or ten weeks and are then returned to short photoperiods, the gonads do not involute a second time. However, if the duration of exposure to LD 14:10 is increased to 22 weeks, the return to LD 1:23 causes the gonads to degenerate. The regressive responses of the testes never occur in hamsters that have been pinealectomized indicating that the observed changes are mediated by this gland. The inability of darkness and, thus the pineal gland, to induce a second gonadal involution unless the hamsters are maintained in LD cycles of 14:10 for 22 weeks (after a 30 week period of dark exposure) may be explicable in terms of (1), a transient refractoriness of the brain to the pineal antigonadotropic principle or (2), the temporary failure of the pineal to be activated by short daily photoperiods.