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Continuous sampling of cerebrospinal fluid (CSF) over 24-h periods in 10 rhesus monkeys revealed a 2-fold, highly reproducible circadian rhythm in CRF concentrations. Peak CRF values of 77.9 +/- 6.4 pg/ml occurred in the evening at 1930 h, while the CRF nadir (38.4 +/- 4.2 pg/ml) occurred at 0745 h. Simultaneously sampled CSF cortisol peaked at 0913 h, with a nadir at 2226 h. Both CRF and cortisol rhythms closely fit sinusoidal circadian models, with r2 values of 0.94 and 0.92, respectively. While hypothalamic CRF is regarded as a major physiological regulator of pituitary ACTH secretion and, thereby, of the circadian and stress-related release of cortisol from the adrenal gland, CRF and CRF receptors are also widely distributed in other brain areas of primates and rodents. The marked difference in the circadian rhythm of CRF vs. that of cortisol suggests that CRF in CSF reflects or mediates some nonhypophysiotropic brain functions of this peptide.

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