title: Evidence for a photoperiod-sensitive pacemaker for estrous cycle of the rat.
abstract: To define how the periodicity of light regulates reproductive cyclicity in the rat, animals were exposed to slightly altered photoperiods in which the light/dark ratio was maintained as in the control (14L + 10D) environment. Exposure to a 22.5-h photoperiod (13.125L + 9.375D) for 10-80 days induced a significant prolongation of the first and all subsequent vaginal cycles by about 30 h. Ovulation in these animals was normal. By contrast, exposure to a 26-h photoperiod (15.167L + 10.833D) had no significant effect on vaginal cyclicity. A comparison of the data with theoreticaly generated results revealed that a significant prolongation of the estrous cycle under conditions of a shortened photoperiod cannot be accounted for by a simple entrainment of the estrous cycle to the timing of the new circadian rhythm. Rather, the evidence suggested that a photoperiod-entrained rhythm may interact with a second rhythm of different periodicity to alter the duration of the reproductive cycle as measured in this study.
mesh_terms: D000818:Animals; D002940:Circadian Rhythm; D003624:Darkness; D004971:Estrus; D005260:Female; D008027:Light; D010063:Ovum / Q000502:physiology; D011247:Pregnancy; D051381:Rats; D014621:Vagina / Q000502:physiology