Abstract
Circadian periodicity of the hypothalamic content of corticotropin-releasing factor (CRF) was determined in normal and adrenalectomized male rats, using the intrapituitary micro-injection method. It was found that the CRF activity in the rat hypothalamus showed a definite circadian variation, having the peak value at 6 p.m. and the minimum at 8 a.m. under the lighting schedule used here (light between 6 a.m. and 7 p.m. followed by 11 h of darkness). A close temporal relationship with a definite phase shift was observed between the CRF activity and plasma corticosterone level. The finding thus supports the concept that the circadian rhythm of the pituitary-adrenal axis is a direct reflection of the rhythmicity of CRF activity in the median eminence. Furthermore, the persistent periodicity observed in the CRF activity in the absence of circulating corticosterone suggests that the dominating mechanism for the control of the circadian rhythm of CRF activity is of neural origin, being independent of the negative feedback mechanism.
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