Narcotic and narcotic antagonist analgesics: their effect on rat brain monoamine oxidase in vitro

Morphine, codeine, normorphine, norcodeine, nalorphine, methadone, pentazocine, cyclazocine, meperidine and levallorphan were tested in vitro for activity as inhibitors of rat brain monoamine oxidase. Beta-phenylisopropylhydrazine and chlorpromazine were used, respectively, as strong and weak inhibitors to define the upper and lower limits of inhibitory activity. All test compounds, with the exception of methadone, were found to be weaker monoamine oxidase inhibitors than chlorpromazine. The log dose-response curve for methadone stretches across four orders of magnitude in concentration and, at low molar concentrations, the monoamine oxidase inhibitory activity of methadone is equivalent to the monoamine oxidase inhibitory activity of betaphenylisopropylhydrazine at equally low concentrations.