Pyridine aldoxime dodecyliodide (PAD) inhibits the monoamine oxidase (MAO) of rat liver and brain in vitro but not in vivo. Other long-chain quaternary compounds are also effective inhibitors in vitro. No preincubation is necessary, and the inhibition is irreversible. PAD is also an effective inhibitor of "solubilized" MAO. PAD lowers the 5-hydroxytryptamine (5-HT) levels in rat brain but has no effect on the catecholamine levels. It does not inhibit 5-hydroxytryptophan decarboxylase of rat kidney but does release 5-HT from mast cells in vitro. These findings suggest that PAD exerts a selective releasing action on bound 5-HT. Animals given sufficient PAD to deplete their 5-HT stores do not have the appearance of animals treated with reserpine. In addition, animals pretreated with phenylisopropylhydrazine (PIH), an inhibitor of MAO, and then given PAD do not have the appearance of animals pretreated with PIH and then given reserpine. Decamethonium and hexamethonium are also weak inhibitors of MAO in vitro.
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