Patterns of reproductive hormone secretion in hibernating Turkish hamsters

Changes in gonadal state and in circulating reproductive hormones [follicle-stimulating hormone (FSH), prolactin (PRL), and testosterone] were studied for 30 wk in male Turkish hamsters (Mesocricetus brandti) induced to hibernate by exposure to a short-day, cold environment [10:14-h light-dark (LD) cycle, 6 +/- 1 degree C]. Similar measures were compared in hamsters maintained under short-day warm conditions (10:14-h LD, 21 +/- 2 degrees C). A decrease in testicular size and in hormone levels was observed after 9-12 wk of short-day exposure in all animals. After 24 wk, hormone levels rose again, accompanied by testicular recrudescence, in short-day warm hamsters and in hamsters that failed to hibernate in the cold. For animals that hibernated the temporal pattern of endocrine and gonadal changes differed only slightly in comparison. Testicular recrudescence of hibernators lagged approximately 3 wk behind that of short-day warm hamsters. Hormone levels were generally lower in hibernators sampled during bouts of torpor than during bouts of spontaneous arousal from torpor. A marked elevation of serum FSH was observed in aroused hibernators well before the end of the hibernation season (at 21 wk of short-day exposure). Mean testosterone and PRL values had increased by wk 27, after hibernation was terminated in the majority of animals. These results indicate that testosterone may not be essential for the termination of the hibernation season. The data also suggest that an endogenous timing mechanism, resistant to the decreased body temperature experienced during torpor, may function to trigger a resurgence of the neuroendocrine-gonadal axis at the end of the winter season.