Modulation of nicotinic receptors by opiate receptor agonists in cultured adrenal chromaffin cells

Abstract

Morphological, physiological and pharmacological evidence indicates that opioid peptides, which in the brain are located intraneurally¹, may function as neurotransmitters. Similar evidence is not yet available for the opioid peptides that are stored in chromaffin cells of adrenal medulla^2–5 and in axon terminals located in adrenal medulla and sympathetic ganglia^3–5. The present report contributes evidence suggesting that the opioid peptides which are stored in the axon terminals of the splanchnic nerves located in adrenal medulla⁴ may function as neuromodulators of the acetylcholine receptors located on chromaffin cells that are involved in catecholamine release. We support this functional role of the opioid peptides by showing that primary cultures of chromaffin cells of bovine medulla contain opiate receptors. When these receptors are occupied by specific agonists, the number of nicotinic receptors and the amount of catecholamine released by maximal doses of nicotine are reduced. Thus, like in other neuronal systems^6–8 also in adrenal medulla, the action of opioid peptides is inhibitory. The specificity of this action is in part supported by the inability of opiate receptor agonists to reduce the Ca²⁺-dependent release of catecholamines elicited by K⁺ ions.