Plasma 5‐Hydroxyindoleacetic Acid as an Indicator of Monoamine Oxidase‐A Inhibition in Rat Brain and Peripheral Tissues

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Abstract
We have examined the changes induced by the monoamine oxidase (MAO; EC 1.4.3.4) inhibitors tranylcypromine, clorgyline, and deprenyl on MAO activity and 5‐hydroxytryptamine (serotonin, 5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) content in rat brain and blood (plasma and whole blood). The decreases of MAO‐A activity observed in the liver and lungs after different doses of clorgyline or tranylcypromine correlated significantly (r > 0.80 in all cases) with the decline of plasma 5‐HIAA. This was unaffected by 0.25 and 5 mg kg−1 of deprenyl, indicating that 5‐HT was deaminated exclusively in the periphery by MAO‐A. It is interesting that very potent and significant correlations (r > 0.75) were found between plasma 5‐HIAA and MAO‐A activity, 5‐HIAA and 5‐HT content in brain tissue. These results suggest that plasma 5‐HIAA can be used confidently as a peripheral indicator of the inhibition of MAO‐A in brain. This may represent a favorable alternative to the analysis of 5‐HIAA in CSF in psychiatric patients undergoing antidepressant treatment with nonspecific MAO inhibitors or with the new selective MAO‐A inhibitors.