Adrenal innervation may be an extrapituitary mechanism able to regulate adrenocortical rhythmicity in rats.

The daily rhythm of plasma corticosteroid concentration was studied in nonhypophysectomized rats and in hypophysectomized rats implanted with beeswax pellets containing ACTH and T4. In the first experiment, male hypophysectomized rats were placed on a 12-h light, 12-h dark cycle (onset of light, 0600 h) with food and water available ad libitum. Beeswax pellets containing 1.5 mg ACTH and 150 micrograms T4 were implanted sc in these rats. Beginning 4 days after the implantation of ACTH and T4, daily rhythms of plasma corticosteroid concentration were detected in these rats on 3 successive days. The injection of sodium pentobarbital (40 mg/kg BW) or atropine sulfate (10 mg/kg BW) ip at 1200 h blocked the expected rise in plasma corticosteroid concentration at 1800 h in both these rats and a group of intact rats. In the second experiment, adrenal innervation was disrupted by spinal cord transection at the T-7 level. L-1-transected rats served as operated controls, and a third group was maintained as unoperated controls. One week after surgery, daily rhythms of plasma corticosteroid concentration were present in both unoperated and L-1 controls (P less than 0.01) but not in the T-7-transected rats. Inasmuch as an extrapituitary mechanism was capable of maintaining adrenocortical rhythmicity in hypophysectomized rats and disruption of adrenal innervation suppressed adrenocortical rhythmicity, it was hypothesized that adrenal innervation may be an extrapituitary mechanism which has a role in adrenocortical rhythmicity.