Role of the muscarinic acetylcholine receptor in parathion toxicity: Chronic acetylcholinesterase inhibition and direct effects

Abstract

Certain muscarinic acetylcholine receptors (mAChRs) have high affinity for organophosphate (OP) anticholinesterases, and their density is reduced by chronic exposure to OPs. The effects of acute in vitro and chronic in vivo exposures to the OP insecticide, parathion, and its more toxic metabolite paraoxon, were studied on mAChRs in the brains and submaxillary gland (SMG) of rats and white-footed mice. Binding of subtype-nonselective and -selective radioligands, receptor functional assays as well as mRNA expression measurements were utilized. In addition to its binding and inhibition of acetyl-cholinesterase (AChE), paraoxon bound directly with even higher affinities to mAChRs. It bound to mAChRs in rat striatum (possibly an M{dollar}\sb4{dollar} subtype) and acted as an agonist, producing atropine-sensitive inhibition of forskolin-activated ({dollar}\sp3{dollar}H) cAMP synthesis. Paraoxon also bound to M{dollar}\sb3{dollar} mAChRs in rat SMG cells, but to a noncompetitive site. Although paraoxon at submicromolar concentration inhibited forskolin-activated ({dollar}\sp3{dollar}H) cAMP synthesis, this was not reversed by atropine. It suggested that paraoxon had two binding sites: an allosteric one on the mAChR, and the other on the G{dollar}\sb{lcub}\rm i{rcub}{dollar} protein-adenylyl cyclase system. The direct high-affinity reversible actions of paraoxon on mAChRs would contribute significantly to its toxicity, especially early on before most AChE is phosphorylated. The majority of mAChRs in the brains of white-footed mice were found to be of the M{dollar}\sb3{dollar} subtype. Adults exposed chronically for 14 days to dietary sublethal doses of parathion had reduced brain AChE activity, and densities of total mAChRs as well as the M{dollar}\sb3{dollar} subtype by {dollar}\approx{dollar}50% without significant change in their affinities. The reduction in mAChRs increased with higher AChE inhibition, which reflected paraoxon concentrations in the brain. The data provide the first evidence for down-regulation of mAChRs in a wild rodent species after dietary exposure to parathion, which would affect their adaptation to environmental exposure to OP insecticides. Furthermore, the changes in mAChRs may provide a molecular basis for certain chronic toxicities of OPs in humans and other mammals.