Effects of substance P on the binding of ligands to nicotinic acetylcholine receptors

GA Weiland, JA Durkin, JM Henley and SM Simasko

Department of Pharmacology, New York State College of Veterinary Medicine, Cornell University, Ithaca 14853.

The effect of substance P on the binding of many ligands that interact with the nicotinic acetylcholine receptor was examined in membrane preparations of Torpedo electroplaque and BC3H-1 cells and in solubilized membranes of rat, chick, and goldfish brain. In the absence of carbamylcholine, the affinity of [3H]phencyclidine for the high affinity local anesthetic binding site on Torpedo membranes was increased by substance P with an EC50 of approximately 5 microM. In the presence of carbamylcholine, which itself increases [3H]phencyclidine binding affinity, substance P caused a decrease in the affinity of [3H]phencyclidine. The concentration dependence of the inhibition, however, was inconsistent with a competitive interaction, since the apparent Hill coefficient was significantly less than one. We conclude from these results that substance P does not directly interact with the high affinity local anesthetic binding site on the nicotinic receptor of Torpedo membranes. Substance P also does not appear to interact directly with the agonist binding site since the peptide had no significant effect on [3H]acetylcholine binding to Torpedo membranes. Substance P inhibited [125I]alpha-bungarotoxin binding to both native and Triton X-100 solubilized Torpedo membranes, although the IC50 was 8- fold higher for the solubilized preparation (12 versus 93 microM). We interpret this inhibition in solubilized membranes as evidence that the peptide may interact directly with a binding site on the nicotinic acetylcholine receptor. Substance P also decreased the initial rate of [125I]alpha-bungarotoxin to membranes prepared from BC3H-1 cells (IC50 = 108 microM) and to solubilized membranes from rat, chick, and goldfish brain. In the brain membranes, however, the peptide did not completely inhibit binding; at the highest concentration examined (100 microM), the maximum inhibition observed was 60%. Consistent with the results for [3H]acetylcholine binding to Torpedo membranes, the peptide had no effect on the binding of the cholinergic agonist [3H](-)nicotine to these tissue preparations. These data suggest that substance P may have a general modulatory action on a subclass of nicotinic receptors that include muscle-type, ganglionic-type, and a putative subpopulation of central nervous system receptors.

Volume 32, Issue 5, pp. 625-632, 11/01/1987
Copyright © 1987 by American Society for Pharmacology and Experimental Therapeutics

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