Postnatal development of circadian rhythm of corticotropin-releasing activity in the rat hypothalamus.

Postnatal development of circadian rhythm of the hypothalamo-pituitary-adrenocortical axis was examined by sacrificing rats twice a day, i. e., 8:30 a. m. and 6:30 p.m. Circadian rhythm of plasma corticosterone level became evident during the third week of life. Hypothalamic content of corticotropin-releasing factor (CRF) was not detectable till the end of the second week after birth. A significant increase of CRF activity became manifested in the evening during the third week of postnatal life, whereas the CRF activity in the morning remained undetectable. Thus, it is apparent that a significant circadian rhythm of hypothalamic CRF activity also begins during the third week of postnatal life in the rat under our experimental conditions. The finding may provide further support for the existence of a circadian rhythm in the hypothalamic CRF content with the peak value in the evening and the nadir in the morning.