Molecular Pharmacology, Vol 20, 511-518, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Interaction of Tricyclic Antidepressants with the Ionic Channel of the Acetylcholine Receptor of Torpedo Electric Organ

¹ Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201

Imipramine (IMIP) inhibited the binding of [³H]perhydrohistrionicotoxin ([³H]H₁₂-HTX) but did not inhibit the binding of [³H]acetylcholine ([³H]ACh) or [³H]d-tubocurarine to Torpedo membranes. Binding of [³H]IMIP to Torpedo, measured by a filter assay, was saturable, reversible, with high affinity (K_d 0.42 µM), and inhibited by channel blockers. In the presence of 20 µM carbamylcholine (carb) the affinity of [³H]IMIP increased almost 4-fold, and the maximal number of binding sites and initial rate of binding increased 5-fold. Preincubation of Torpedo membranes with carb reduced this agonist-stimulated binding of [³H]IMIP. [³H]IMIP binding was stimulated by other agonists as well as by the antagonist d-tubocurarine, although to a much lower degree than agonists, and these increases were totally blocked by -bungarotoxin. [³H]IMIP binding to Torpedo membranes differed in two major aspects from [³H]H₁₂-HTX binding. Its maximal binding in absence of carb was much lower than in its presence, and binding was little affected by changes in temperature, more like binding of receptor agonists which showed weak temperature dependency. It is suggested that [³H]IMIP binds to sites on the ionic channel moiety of the receptor/channel molecules which are allosterically coupled to the ACh binding sites, but there are differences between binding of [³H]IMIP and [³H]H₁₂-HTX. The changes in [³H]IMIP binding are suggested to be due to ligand-induced conformations of the ACh-receptor/channel molecule. Six tricyclic antidepressants (IMIP, nortriptyline, amitriptyline, desimipramine, protriptyline, and doxepin) blocked [³H]H₁₂-HTX and [³H] IMIP binding with slightly differing potencies, with doxepin as the least potent. It is suggested that this nicotinic ACh receptor/channel molecule is not a primary target for tricyclic antidepressant drugs or their cardiotoxicity, although similar ionic channels or carriers may be.

Note:
ACKNOWLEDGMENTS We are grateful to Dr. John Daly of the National Institutes of Health for kindly providing us with [³H]H₁₂-HTX; our colleague Dr. Edson X. Albuquerque for protriptyline, desimipramine, and nortriptyline; Mr. Charles A. Brownley, Jr. (Geigy Pharmaceuticals), Dr. Frank A. Cutler (Merck Sharp & Dohme), and Dr. Eugene M. Weiss (Pfizer Laboratories) for IMIP, amitriptyline, and doxepin, respectively.