Substance P enhances cholinergic receptor desensitization in a clonal nerve cell line.

Abstract

The PC12 [neoplastic rat adrenal pheochromocytoma] cell line has surface nicotinic acetylcholine receptors that closely resemble those found on sympathetic neurons and Renshaw cells. Many of the functional properties of these receptors can be investigated by using a 22Na+ influx assay to measure receptor activation. We used these cells to examine the effects of substance P on activation of neuronal nicotinic acetylcholine receptors. Substance P inhibits carbamylcholine-induced 22Na+ uptake in the PC12 cells. This inhibition is noncompetitive with agonist but competitive with Na+. Octahydrohistrionicotoxin (H8-HTX) also exhibits this same pattern of inhibition. Both substance P and H8-HTX are very effective in enhancing agonist-induced receptor desensitization. Local anesthetics, such as QX222 [2-(trimethylamino)-N-2,6-(dimethylphenylacetamide)], also cause inhibition that is competitive with Na+, but they have only marginal effects on desensitization. Because substance P and H8-HTX cannot themselves cause desensitization, their action is dependent on and synergistic with the action of agonist. Substance P and H8-HTX do not appear to compete for the same site as QX222, which is thought to bind to the ion channel. Substance P can stabilize the desensitized state of the receptor even when added subsequent to the actual desensitization and removal of agonist. Substance P does not require open ion channels for binding and may modulate the activity of the receptor-ionophore complex by binding to a distinct regulatory site.