Evaluation of the inhibitory effect of circulating corticosteroids on circadian stimulated and stress induced secretion of ACTH in rats

Male rats were bilaterally adrenalectomized in order to measure the extent of inhibition exerted by endogenous corticosteroids on both basal ACTH secretion along its circadian rhythm and ether-stress induced ACTH secretion. In intact controls, plasma ACTH levels at the circadian maximum exceeded by 4 times the circadian minimum, and ACTH response 15 min after ether-stress surpassed the circadian minimum by 20 times. In adrenalectomized rats, the daily minimum was 8 times that of the controls. Nevertheless the circadian maximum was 3 times above the rhythm's minimum, while the maximal stress response (15 min) surpassed the circadian minimum by 8 times. In adrenalectomized rats supplemented with a solid source of corticosterone inducing a stable plasma corticosterone level equivalent to the controls' circadian minimum (3 micrograms/100 ml), the ACTH rhythm still fluctuated twice as high as in intact controls. The tonic feed-back inhibition exerted by endogenous corticosteroids on ACTH secretion appeared thus significantly stronger than the GABAergic inhibition to the corticotropic system which was previously studied under similar standard conditions.