Circadian rhythm in c-fos protein expression in the rat adrenal cortex

The circadian variation in the expression of c-fos protein(s) in the adrenal cortex of adult rats was investigated immunocytochemically. The animals were maintained in standard laboratory facilities with a 12:12 h light:dark schedule (07.00-19.00 h) until sacrificed. The number of c-fos immunoreactive (c-fos IR) cells was statistically constant in the zona fasciculata and zona reticulata at 20.00 h and 24.00 h, and 04.00 h, but was decreased to 52% at 08.00 h and to 18% at 12.00 h, respectively, when compared to the 24.00 h value. At 16.00 h, the number of c-fos IR cells was increased again to 63% of the 24.00 h value. In the zona granulosa c-fos IR was observed only at 20.00 h and 24.00 h. Administration of dexamethasone, a potent inhibitor of ACTH release, significantly reduced the number of c-fos IR cells. As the circadian rhythm of c-fos expression in the adrenal cortex correlates to that reported for ACTH secretion from the pituitary, the results suggest that c-fos gene is involved in mediating physiological ACTH-induced responses in the adrenal cortical cells.