Corticotropin-releasing factor concentrations exhibit an apparent diurnal rhythm in hypothalamic and extrahypothalamic brain regions: differential sensitivity to corticosterone.

Regional brain concentrations of corticotropin-releasing factor (CRF) exhibited marked differences in a number of regions depending upon the time of day rats were sacrificed. When compared with animals killed at 09.00 h, CRF concentrations at 15.30 h were elevated in the median eminence, hypothalamus minus median eminence, preoptic area/suprachiasmatic nucleus, bed nucleus of the stria terminalis, septum, frontal/parietal cortex, cerebellum, cingulate cortex, locus ceruleus and the nucleus of the solitary tract. Animals that received 7 days of continuous corticosterone supplementation via osmotic minipump either did not exhibit an afternoon rise in CRF concentrations (median eminence, cingulate cortex and locus ceruleus) or exhibited significantly attenuated afternoon increases in CRF concentrations (hypothalamus minus median eminence). In rats killed at 09.00 h, corticosterone reduced CRF concentrations in the median eminence. Chronic corticosterone exposure did not affect CRF concentrations in any of the extrahypothalamic brain regions studied in rats killed at 09.00 h. These results suggest that a number of CRF-containing neurons in both hypothalamic and extrahypothalamic brain regions presumably undergo diurnal changes in their activity as evidenced by changes in CRF concentrations. Furthermore, chronic corticosterone supplementation can alter these changes, particularly those in the endocrine hypothalamus.