Molecular Pharmacology, Vol 17, 24-30, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Altered [³H]Quinuclidinyl Benzilate Binding in the Striatum of Rats following Chronic Cholinesterase Inhibition with Diisopropylfluorophosphate

¹ Department of Medical Pharmacology and Therapeutics, University of California College of Medicine, Irvine, California 92717

Chronic administration of diisopropylfluorophosphate to rats at a dose regimen producing 90% inhibition of striatal cholinesterase caused a significant decrease in the density of [³H]quinuclidinyl benzilate binding sites in homogenates of the striatum. The concentration of binding sites decreased from 1.39 ± 0.06 in controls to 0.82 ± 0.02 pmol/mg protein in diisopropylfluorophosphate-treated rats, and the half-time for the loss of binding sites was approximately 1.6 days. This decrease in [³H]quinuclidinyl benzilate binding was not due to a direct effect of diisopropylfluorophosphate on muscarinic receptors since no inhibition of binding was produced by high concentrations of diisopropylfluorophosphate when added in vitro. An increase in the K_i for inhibition of [³H]quinuclidinyl benzilate binding by various nonlabeled cholinergic ligands was observed in chronic diisopropylfluorophosphate-treated rats, indicating that the affinity of the muscarinic receptors in the striatum had decreased. Following diisopropylfluorophosphate treatment, muscarinic receptors displayed a greater decrease in affinity to muscarinic agonists than to antagonists. Scatchard analysis of oxotremorine inhibition of [³H]quinuclidinyl benzilate binding showed both high (K_H = 0.01 ± 0.002 µM) and low (K_L = 0.86 ± 0.18 µM) affinity binding sites in relative densities of 26 and 74%, respectively. Chronic administration of diisopropylfluorophosphate produced an increase of K_H to 0.037 ± 0.008 µM, indicating a decrease in affinity but had no significant effects on K_L or on the relative densities of the two sites. Similar results were obtained for acetylcholine and pilocarpine. In contrast, Scatchard analysis of the atropine inhibition of [³H]quinuclidinyl benzilate binding showed a single class of antagonist binding sites. These decreases in affinity and density of muscarinic receptors in the striatum following chronic cholinesterase inhibition demonstrate that the tolerance to diisopropylfluorophosphate is, in part, a receptor mediated phenomenon.