Differential effects of certain dopaminergic drugs on the striatal concentration of dopamine metabolites, with special reference to 3-methoxytramine.

Studies were undertaken to evaluate the effect of certain dopamine (DA) agonists and antagonists on DA metabolite concentrations in rat striatum, with special regard to 3-methoxytyramine (3-MT). Quipazine, nomifensine and piribedil act as dopaminergic agonist drugs. However piribedil, which is a dopaminergic receptor agonist, reduced the concentrations of all 3 metabolites considered, while nomifensine produced an increase, and quipazine caused an early rise in 3-MT followed by a lowering of the concentration of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Haloperidol and clozapine are both DA antagonists. Haloperidol, which blocks DA receptors, caused an increase in the formation of all 3 metabolites while clozapine raised DOPA and HAV but not 3-MT. The importance of simultaneous determination of these 3 metabolites is discussed with relation to evaluation of the mechanism of action of these drugs and - more generally - of drugs acting on the dopaminergic system.