A Sensitive Enzymatic‐radioisotopic Assay for Apomorphine

An assay for the dopaminergic agonist, apomorphine, is described. The assay is capable of detecting as little as 3 pmol of apomorphine. The basis of the assay is the O‐methylation of apomorphine utilizing S‐[methyl‐ 3 H]adenosyl‐L‐methionine and a partially purified catechol‐O‐methyl transferase to form [methyl‐ 3 H]apocodeine. The radiolabeled apocodeine is purified by solvent partition. Following systemic administration of apomorphine, the compound is uniformly distributed throughout the brain of the rat: there is no selective accumulation of apomorphine in dopaminergic regions of the brain. The level of apomorphine found in the substantia nigra is as great as the level found in the caudate‐putamen. This latter observation, in addition to a variety of other evidence, raises the possibility that some of the in vivo effects of apomorphine may occur via an action of the drug within the substantia nigra, rather than by an action within the caudate‐putamen.