Extrapituitary Regulation of the Circadian Rhythm of Plasma Corticosteroid Concentration in Rats*

Intravascular injections of adrenocorticotropic hormone in hypophysectomized rats were approximately 3 times more effective in stimulating increases in plasma corticosteroid concentration when given at the offset rather than the onset of the daily photoperiod (12 h:12 h light-dark cycle). Reversal of the photoperiodic schedule produced a reversal in the corticosteroid response after 5 days. Similar temporal variations in androgen and T4 responses to LH and TSH were not obtained. Although plasma corticosteroid concentrations were maintained at normal intermediate levels in rats with adrenal autotransplants, daily variations were not demonstrable. These results indicate that the circadian rhythm of plasma corticosteroid concentration is regulated by way of neural pathways to the adrenal.