Effects of Bilateral Adrenalectomy and Continuous Light on the Circadian Rhythm of Corticotropin in Female Rats1

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Abstract
The rhythm of plasma corticosterone in intact female rats exposed to constant light for 8 weeks was compared with that in a control group exposed to alternating light-darkness corticosterone levels were depressed and the peak ablated in the experimental group. The daily rhythm of corticotropin in plasma and pituitary tissue was measured by bioassay in 3 groups of adult female adrenalectomized rats. The first group was exposed to alternating light for 7 days and the second and third to alternating light and to constant light, respectively, for 8 weeks. A definite rhythm of corticotropin secretion was observed and the peak value appeared at 1 PM (1300 hr) in the 7-day adrenalectomy group. Constant light was associated with a diminution of plasma levels and a suppression of the rhythmic secretion of corticotropin. The results suggest a dominating neural mechanism for the control of the circadian rhythm of corticotropin, which is depressed by exposure to constant light.