Secretion of follicle-stimulating hormone and luteinizing hormone in castrated golden hamsters during exposure to various photoperiods and to natural daylengths

Golden hamsters were castrated and either maintained under short days of 9 h light and 15 h darkness (9L:15D) or transferred to 12L:12D, 13L:11D or 16L:8D. Plasma concentrations of FSH and LH remained low under 9L:15D and 12L:12D for 6-8 weeks but rose markedly within 4 weeks under 13L:11D and 16L:8D, suggesting a more abrupt transition between non-stimulatory and maximally stimulatory photoperiods than found in Japanese quail. Intact and castrated hamsters were exposed to natural photoperiods at a latitude of 51° 27' N (Bristol) for 12 months. In the intact animals plasma FSH and LH levels and the size of the testes decreased as the daylength shortened from 12.5 to 10.5 h during autumn. This was followed by a rise in FSH output in mid-winter with an upward trend in LH secretion. The ensuing testicular recrudescence was complete before the spring equinox. The results emphasize that under natural conditions the primary factor regulating gonadal growth is the development of refractoriness to short days. In castrated hamsters there was no significant seasonal trend in LH output. Levels of FSH remained unchanged until a peak occurred in January, the same time that a peak was seen in intact animals before testicular recrudescence. A similar peak occurred in castrated hamsters maintained in 9L:15D in the laboratory. This suggests that even in the absence of gonadal steroid negative feedback hypothalamo-pituitary activity changes during the photorefractory period.