Multiple Transmitter Control of Catecholamine Secretion in Rat Adrenal Medulla

Abstract

Generally it is considered that excitation of splanchnic neurons during exercise and stress and so on, releases acetylcholine (ACh), which then activates the cholinergic receptors of the chromaffin cells to stimulate secretion of catecholamines (CAs) and other biochemical functions of chromaffin cells; but a number of recent studies suggests that there is convincing new evidence that in addition to ACh, several peptides released in the adrenal medullary synapse are capable of affecting CA secretion, synthesis, and differentiation of chromaffin cells. This chapter briefly covers new evidence about peptidergic innervation of the rat adrenal medulla and the contribution of the peptides released from these neurons in the secretion and synthesis of CA. A large number of experiments have been carried out in isolated perfused adrenal gland of rat. A distinct advantage of this preparation is that secretion of CA could be evoked by excitation of splanchnic neurons as well as exogenous ACh. If ACh is the only transmitter involved in the stimulation of CA secretion, then it can be anticipated that antagonists of nicotinic and muscarinic receptors would block the secretion evoked by splanchnic nerve stimulation. However, in study secretion evoked by splanchnic nerve stimulation is reduced but not blocked. This substantial secretion in the face of effective cholinergic block has been taken as a first indication that some noncholinergic transmitter released from neurons innervating the adrenal medulla is responsible for the secretion. The data also hinted that the contribution of noncholinergic substance is more prominent at lower than higher frequencies of stimulation.