Circadian rhythmicity of ACTH and corticosterone in the rat

The circadian secretion of ACTH and corticosterone was assessed by measuring immunoreactive ACTH concentration in the plasma and ACTH content in the anterior and posterior pituitary over a 30 h period in groups of both male and female rats and comparing these data to fluorometric corticosterone concentration in the plasma and corticosterone content in the adrenal. A circadian rhythm of plasma corticosterone levels and adrenal content was apparent in both males and females with the highest levels at the onset of darkness. In contrast, there was no significant circadian rhythm in plasma ACTH levels or anterior or posterior ACTH pituitary content. Because the ACTH and corticosterone rhythms were dissociated, rhythmic corticosterone secretion is not entirely dependent on ACTH secretion.