Abstract
Diurnal periodicity of the hypothalamic content of corticotropin-releasing factor (CRF) was determined in the normal male rat by the intrapituitary microinjection method. It was found that the CRF activity in the rat hypothalamus showed a definite diurnal variation, having the peak value at about 18: 00 and the minimum at 4: 00 or 8: 00 under the lighting regimen of 6: 00 to 19: 00 of day light followed by 11 hrs. of darkness. The close temporal relationship with a definite phase shift observed between CRF activity and plasma corticosterone level appears to support the concept that the diurnal rhythm of the pituitary-adrenal axis is a direct reflection of the rhythmicity of CRF activity in the median eminence of the rat.
