Regulation of acetylcholine receptors by endogenous cotransmitters: studies of adrenal medulla

The coexistence of neuropeptides and amine transmitters in the same neuron and the presence of postsynaptic receptors for these compounds invite speculation that the compounds may interact postsynaptically. To study this interaction, we selected the synapse between the splanchnic nerve and the chromaffin cells of adrenal medulla because the splanchnic nerve contains acetylcholine and neuropeptides (multiple molecular forms of enkephalin-like peptides), while the membranes of chromaffin cells contain receptors for acetylcholine and opiate peptides. When the opiate receptors are occupied by specific agonists, these agonists inhibit the expression of acetylcholine receptors. Both the acetylcholine binding and catecholamine release by acetylcholine were inhibited. It is suggested that peptides and acetylcholine coexisting in the splanchnic nerve might act as cotransmitters, there by modulating the sensitivity of acetylcholine receptors. Multiple molecular forms of enkephalin-like peptides are stored in chromaffin cells and are released by acetylcholine. A role for the opiate receptors in the modulation of the secretion of opiate peptides stored in chromaffin cells is not evident.