Differential modulation of organophosphate-sensitive muscarinic receptors in rat brain by parathion and chlorpyrifos.

We previously reported similar levels of brain cholinesterase inhibition but marked differences in toxicity following acute maximum tolerated doses of the organophosphate pesticides parathion and chlorpyrifos. Because extensive acetylcholinesterase inhibition often induces compensatory changes in cholinergic receptor populations, we compared the effects of parathion and chlorpyrifos on brain muscarinic receptors. Adult male rats were treated with vehicle or the maximum tolerated dose of parathion (18 mg/kg, sc) or chlorpyrifos (279 mg/kg, sc) and observed for signs of acute toxicity. Similarly treated animals were sacrificed at 2, 7, or 14 days after treatment for measurement of cholinesterase activity and binding to the nonselective muscarinic antagonist [3H]quinuclidinyl benzilate, the M2-preferential antagonist [3H]AFDX-384, and the high-affinity agonist [3H]cis-methyldioxolane. More acute toxicity was noted after parathion treatment. Both insecticides caused similar levels (> 85%) of maximal cholinesterase inhibition and reductions (up to 55%) in atropine-sensitive quinuclidinyl benzilate binding (i.e., total muscarinic receptors) and [3H]AFDX-384 binding in cortex and striatum. Parathion also reduced, whereas chlorpyrifos increased, total muscarinic receptor binding and [3H]AFDX-384 binding in the cerebellum. When tissues were preincubated with paraoxon (10 microM), radiolabeling of a subset of quinuclidinyl benzilate binding sites was blocked and the apparent densities of these organophosphate-sensitive receptors in all three tissues were decreased (16% maximal) by parathion but increased (up to 37%) by chlorpyrifos. Similarly, parathion decreased whereas chlorpyrifos increased [3H]cis-methyldioxolane binding sites in all three brain regions. We propose that differential modulation of these organophosphate-sensitive muscarinic receptors contributes to differences in acute toxicity following exposure to these pesticides.