The Effect of Constant Illumination on the Circadian Rhythms of Plasma Thyrotropin and Corticosterone and on the Estrous Cycle in the Rat

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Abstract
Young adult female rats were kept on a normal light/dark (LD) schedule (12 h light beginning at 0600 h) for 10 days. Eighteen rats were then kept in constant light (LL) for 110 days; 9 controls were maintained on LD. All rats were then kept on LD for a further 30 days. Vaginal smears were made 5 days per week. Plasma TSH [thyrotropin] and corticosterone were measured on consecutive days at 1100-1130 h and 1745-1815 h. LD controls maintained normal rhythms of plasma TSH and corticosterone and of estrus throughout the experiment. After beginning LL, plasma corticosterone and TSH AM-PM differences in group means disappeared within 1 wk, while constant estrus did not appear until approximately 4 wk. Marked AM-PM differences in plasma hormone concentration were present in individual LL rats. Within 1 wk of the return to LD, normal estrous, TSH and corticosterone rhythms had resumed. There was a significant negative correlation of plasma TSH and corticosterone (P < .02) during LD but not during LL (P > 0.1). From these and other data, it is concluded that a free-running circadian rhythm of plasma corticosterone, and probably of TSH, occurs within a few days of transfer from LD to LL. These rhythms are independent of each other, indicating different pacemakers for each. Both free-run in LL before the estrous cycle is lost. In spite of prolonged free-running, entrainment of all 3 cycles resumes promptly when LD cues are restored.