Effects of organophosphates and nerve growth factor on muscarinic receptor binding number in rat pheochromocytoma PC12 cells

Muscarinic receptor binding in PC12 cells is influenced by both nerve growth factor (NGF) and organophosphates. Treatment of PC12 cells with a single dose of NGF (50 ng, 7S NGF/ml) increased 3H]N-methylscopolamine (3H]-NMS) binding sites approximately two-fold at 48 hr but did not change the Kd for this ligand. Exposure of PC12 cells to soman, 50 microM, decreased 3H]-NMS binding in both undifferentiated and NGF-treated cells; however, decreases in muscarinic binding induced by the organophosphate were only minimal after the first hour after treatment and were maximal at about 24 hr. Other organophosphates including sarin, tabun, and VX as well as the carbamate, pyridostigmine, also reduced 3H]-NMS binding in PC12 cells measured 24-48 hr after treatment. The order of potency of organophosphates in lowering 3H]-NMS binding was soman greater than sarin greater than VX greater than tabun greater than DFP. High amounts of VX (200 microM) but not the other organophosphates inhibited 3H]-NMS binding when added to cells during the course of binding assays. Decreases in muscarinic receptor binding induced by the organophosphates differed markedly from that produced by carbamylcholine, which decreased 3H]-NMS binding maximally 30 min after addition to the cells. Decreases in 3H]-NMS binding produced by carbamylcholine were antagonized by atropine, but reductions in muscarinic binding produced by the organophosphates were not reversed by atropine. Thus, decreases in muscarinic receptor binding induced in PC12 cells by organophosphates occur via a novel mechanism that does not involve agonist-induced receptor desensitization.