Effects of chlorpyrifos oxon on M2 muscarinic receptor internalization in different cell types

The muscarinic M2 receptor is a member of the G protein-coupled receptor (GPCR) superfamily. Agonist activation of GPCR leads to their phosphorylation, desensitization, internalization, and subsequent endocytic recycling or lysosomal degradation. Agonist-induced phosphorylation of M2 receptors is mediated by G-protein receptor kinase 2 (GRK2). The active metabolite of the organophosphorus insecticide chlorpyrifos, i.e., chlorpyrifos oxon (CPO), inhibited agonist-induced phosphorylation of human recombinant M2 receptors by GRK2 in vitro in a concentration-dependent manner. In both intact HEL 299 cells (human embryonic lung fibroblasts expressing M2 receptors) and CHO-M2 cells (stably expressing M2 receptors), the muscarinic agonist carbachol (100 microM) led to receptor internalization as determined by reduced specific binding to the membrane-impermeable radioligand [(3)H]-N-methylscopolamine (NMS). CPO alone (100 microM) exerted no significant effect on NMS binding in either HEL 299 or CHO-M2 cells. In HEL 299 cells, CPO did not influence carbachol-induced internalization, whereas in CHO-M2 cells CPO blocked internalization. In primary striatal neurons, M2 receptors appeared widely and diffusely distributed. Exposure to either carbachol or CPO led to apparent receptor internalization with an increased percent of cells exhibiting punctate domains of immunostaining, while combined exposure to both carbachol and CPO led to a significantly higher percent of cells exhibiting this appearance. The data suggest that CPO may differentially influence agonist-stimulated M2 receptor internalization in a cell-dependent manner.