Abstract

Tetrahydroaminoacridine (THA) is known to be a potent centrally acting cholinesterase inhibitor. In this report, the effects of THA in vivo and in vitro on the binding of muscarinic agonists and antagonists to putative M1 and M2 receptor subtypes were assessed in rat brain membranes. THA competitively inhibited labeled agonist and antagonist binding to membranes prepared from M1 and M2 enriched brain regions. The dissociation of radiolabeled antagonists from muscarinic receptors was decelerated markedly by THA. The half-time for dissociation of [3H]oxotremorine-M from the high affinity state of M1 and M2 receptors was unaffected by THA. Chronic THA administration resulted in a selective down regulation in the number of M1 receptors assayed directly with the M1-selective antagonist, [3H]pirenzepine. The decrease in the binding capacity of [3H]pirenzepine was correlated positively with the duration of drug treatment. Saturation analysis of [3H]pirenzepine binding confirmed that this loss in binding capacity was due to a reduction in the number of binding sites and not an altered affinity of the receptor for [3H]pirenzepine. Carbachol-[3H]pirenzepine competition revealed no change in the ratio of high and low affinity agonist states of the M1 receptor with chronic THA administration. In vivo studies demonstrate further that the total number of muscarinic receptors was decreased significantly, whereas putative M2 receptors, measured directly with the agonist [3H]oxotremorine-M or estimated by pirenzepine-[3H]quinuclidinyl benzilate competition, were unchanged. Thus, THA exhibits multiple actions at primary and secondary recognition sites on putative M1 and M2 subclasses of muscarinic receptors. The results suggest further that the clinical pharmacology of THA may represent a composite efficacy of THA at multiple sites on cholinergic synapses.