High-affinity activation by paraoxon of a muscarinic receptor subtype in rat brain striatum

The mechanism of action of the anticholinesterase paraoxon on the function of a muscarinic receptor subtype in rat brain striatum was investigated. Paraoxon inhibited binding of the muscarinic agonist cis -[ 3 H]methyldioxolane, which binds to a high-affinity population of muscarinic receptors, with K 0.5 of 80 n M , compared to a K 0.5 of 7 μ M for parathion. The inhibition was competitive, suggesting that paraoxon and CD bind to a common site. When this muscarinic receptor (possibly an M 4 subtype) is activated it inhibits cAMP synthesis. Thus, function of the paraoxon-sensitive receptor was assayed by the inhibition of the forskolin-activated [ 3 H]cAMP synthesis. Paraoxon inhibited cAMP synthesis in a dose-dependent manner as did the muscarinic agonist carbachol, and this inhibition was completely blocked by the muscarinic antagonist atropine. When low concentrations of carbachol and paraoxon were used together, there was additive inhibition of cAMP synthesis. However, there was no further increase when both paraoxon and carbachol were present in concentrations that individually produced maximal inhibition. The data suggest that paraoxon acts like carbachol, causing activation of the muscarinic receptor subtype in brain striatum and leading to inhibition of cAMP synthesis. Considering the high affinity that this muscarinic receptor has for paraoxon, it is suggested that this direct reversible action of paraoxon on the muscarinic receptor could affects its toxicity, expecially early on before most of the acetylcholinesterase is phosphorylated.