Figure 1.

Circadian control of adrenal catecholamine output. Photic signals entrain the body's master clock in the hypothalamus (SCN), sending oscillating signals to the PVN. The PVN propagates these neural signals to the pituitary gland via various neurotransmitters and hormonal messengers. The pituitary gland responds by releasing ACTH, which drives adrenocortical cells in the adrenal gland to produce glucocorticoids. These signal to chromaffin cells in the medulla, activating the enzyme, PNMT, which catalyzes the conversion of NE stores into EP for release into the bloodstream. The adrenal medulla is also innervated by acetylcholine (ACh)-producing neurons which also respond to cyclic light signals in the SCN. Stimulation of these neurons activates chromaffin cells, driving the release of NE into the bloodstream in a cyclic fashion.