Circadian pacemaker times gonadotropin release in free-running female hamsters.

Female golden hamsters (Mesocricetus auratus) were housed individually on a photoperiod of LD 6:18 (lights 1000-1600 h). Estrous cyclicity was interrupted for an average of 16 wk after which cycles resumed spontaneously. Such animals are photorefractory, remaining cyclic on normally nonstimulatory photoperiods. Photorefractory females were exposed to continuous darkness in which estrous cyclicity in the population rapidly became asynchronous as each hamster's "day" assumed the endogenous periodicity (tau) of her circadian clock. Tau usually approximated but rarely equaled 24 h. Successive estrous cycles in each animal possessed a periodicity of 4 tau. Preovulatory luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release occurred at a specific time of the animal's circadian day, bearing a discrete phase relationship of 2 to 3 h to the circadian rhythm of locomotor activity. Thus, although seemingly asynchronous with respect to laboratory time, peak preovulatory gonadotropin release on proestrus in free-running hamsters was synchronized with respsect to circadian time. We conclude that a circadian pacemaker times preovulatory gonadotropin release in hamsters.