A new method for in vivo measurement of brain monoamine oxidase activity.

The radiotracers, C-14-N-methylphenylethylamine (MPEA) and N-methylphenylethanolamine (MPEOA) both rapidly entered mouse brain after their intravenous injection and were metabolized by brain monoamine oxidase (MAO) to C-14-methylamine and corresponding aldehydes. The labelled metabolite was trapped in the brain. Measurement of radioactivity showed that the amount of the metabolite produced in the brain from C-14-MPEA was proportional to the MAO activity remaining after combined treatment with a specific MAO-A inhibitor, clorgyline and a MAO-B inhibitor, 1-deprenyl, but not by treatment with either inhibitor alone. The rate of production of the labelled metabolite produced from C-14-MPEOA was highly sensitive to the extent of inhibition of MAO-B activity (with phenylethylamine as substrate) by pretreatment with 1-deprenyl, but was relatively insensitive to inhibitor clorgyline. This selectivity suggests that MPEOA is a specific substrate of MAO-B in mouse brain in vivo. The above results indicate that C-14-labelled N-methylphenylethylamine and N-methylphenylethanolamine derivatives can be used for measurement of brain MAO activity and that C-14-MPEOA is a specific substrate for mouse brain MAO-B. The value and possible applications of this method for measurement of MAO-B in brain under different physiological conditions are discussed.