Chlorpyrifos oxon binds directly to muscarinic receptors and inhibits cAMP accumulation in rat striatum

Abstract

Although the acute effects of organophosphorus esters are generally ascribed to inhibition of acetylcholinesterase, work in this laboratory and others indicates that organophosphorus insecticides also interact directly with cholinergic receptors. The current study verifies that the insecticide O,O-diethyl O-3,5,6-trichloro-2-pyridinyl phosphorothionate (chlorpyrifos) and its oxon metabolite inhibits acetylcholinesterase (AChE). The metabolite inhibits rat brain AChE three orders of magnitude more rapidly than chlorpyrifos. In addition to their ability to inhibit AChE, these compounds were shown to interact directly with muscarinic receptors of rat striatum. The oxon metabolite bound at low concentrations to muscarinic receptors labeled by the muscarinic agonist [3H] cis-methyldioxolane; chlorpyrifos oxon bound with an IC50 value of 22.1 +/- 3.6 nM. The receptors bound by chlorpyrifos oxon account for approximately 30% of muscarinic receptors of the striatum and are of the m2 subtype. The binding of chlorpyrifos oxon to the m2 receptor results in a covalent modification of the receptor that does not interfere with the ability of the receptor to interact with the agonist carbachol. This receptor modification may be responsible for the inhibition of adenylate cyclase activity by chlorpyrifos oxon. The oxon inhibited adenylate cyclase with an IC50 of 155 +/- 78 nM. The inhibition of adenylate cyclase activity was not blocked by atropine and was additive to that produced by carbachol. The altering of postreceptor signal transduction by chlorpyrifos oxon may interfere with normal cellular signaling, thereby disturbing neurological function. Direct interaction of chlorpyrifos oxon with muscarinic receptors and associated signal transduction is a potential mechanism of neurotoxicity that is independent of AChE inhibition.