AJP - Regulatory, Integrative and Comparative Physiology, Vol 238, Issue 1 23-R27, Copyright © 1980 by American Physiological Society
Circadian pacemaker times gonadotropin release in free-running female hamsters
M. H. Stetson and P. J. Anderson
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.
