Abstract
It is well known that there is a marked daily rhythm in the responsiveness of pituitary-adrenal axis to the stress in man. This phenomenon suggests the fact that the higher levels of corticoids in plasma may have had an inhibitory effect on the ACTH release induced by the administered stress. Thus we tried to examine in what degree the CRF activity in the median eminence in response to the stimulus was influenced by corticoids from the adrenal gland and ACTH from the pituitary. In our experiment, a circadian rhythm in the increase of CRF activity after the stress from the non-stress level was observed in normal rats and still persisted in adrenalectomized and hypophysectomized rats. Furthermore, the level of CRF activity attained after the stress at the time when CRF activity in the circadian rhythm was highest was similar to that at the time when it was lowest, in normal, adrenalectomized and hypophysectomized rats. Thus it is likely that the response of ACTH release to stimulus is not influenced by the diurnal rhythm of plasma corticoids or ACTH, but is related to other factors such as inherent neural sensitivity to stress.
