The estrous cycle in golden hamsters: A circadian pacemaker times preovulatory gonadotropin release

In a population of cycling female hamsters entrained to an LD 6:18 light cycle (lights 1000-1600 hours), preovulatory release of luteinizing hormone and follicle-stimulating hormone occurred in some animals at 1300-1400 hours and in others at 1900 hours. In every case peak release was phase-locked (2-3-hour positive phase angle) to the circadian rhythm of locomotor activity. The pattern of entrainment of gonadotropin release on LD 6:18 is fully explicable in terms of the hamster's phase response curve to light. We conclude that periodic gonadotropin release in cycling females is timed by a circadian oscillator (biological clock) that is probably the same oscillator driving the circadian rhythm of locomotor activity.