Action of an irreversible acetylcholine esterase inhibitor, soman, on muscarinic hyperpolarization in cat bladder parasympathetic ganglia

1 Intracellular recording techniques were used to examine the action of an irreversible acetylcholine esterase (AChE) inhibitor, soman, on the hyperpolarizations mediated through muscarinic cholinoceptors in cat bladder parasympathetic neurones. 2 Soman (0.1–10 μm) depressed the amplitude and prolonged the duration of the muscarinic slow inhibitory postsynaptic potential (s‐i.p.s.p.) elicited by a preganglionic tetanus (40 Hz for 1 s) in the presence of mecamylamine (20 μm), phentolamine (1 μm) and caffeine (1 mm), in a dose‐dependent manner. The effect of soman on the amplitude of the s‐i.p.s.p. was partially reversible, while the effect on the duration was irreversible. 3 Soman hyperpolarized the membrane and decreased input resistance, but this effect could not account for soman‐induced inhibition of the s‐i.p.s.p. 4 Soman depressed the amplitude and prolonged the duration of a muscarinic hyperpolarization induced by pressure application of acetylcholine (ACh) in the presence of mecamylamine, phentolamine and caffeine. The time course of this effect paralleled that on the synaptically‐evoked muscarinic s‐i.p.s.p. 5 A reversible AChE inhibitor, pyridostigmine (10–100 μm), also depressed the amplitude and prolonged the duration of a muscarinic hyperpolarization induced by either preganglionic stimulation or ACh pressure application. These actions were reversible, and not accompanied by a significant change in membrane potential or input resistance. 6 The inhibitory action of soman (1 μm) on the muscarinic hyperpolarization was prevented by pyridostigmine (10 μm), but not by atropine (1 μm). 7 These results demonstrate that soman prolongs not only the muscarinic hyperpolarization, but also inhibits its amplitude through a postsynaptic action, probably through AChE inhibition, in cat bladder parasympathetic neurones.