Different action on dopamine catabolic pathways of two endogenous 1,2,3,4‐tetrahydroisoquinolines with similar antidopaminergic properties

The effect of single and multiple 1‐methyl‐1,2,3,4‐tetrahydroisoquinoline (1MeTIQ) and 1‐benzyl‐1,2,3,4‐tetrahydroisoquinoline (1BnTIQ) administration on concentrations of dopamine and its metabolites: homovanillic acid (HVA) and 3,4‐dihydroxyphenylacetic acid (DOPAC) and 3‐methoxytyramine (3MT) in three brain areas was studied HPLC with electrochemical detection in Wistar rats. The rate of dopamine catabolism in the striatum along the N‐oxidative and O‐methylation pathways was assessed by calculation of the ratio of appropriate metabolites to dopamine concentration. In addition, the spontaneous and apomorphine‐stimulated locomotor activity, and muscle rigidity was studied after acute administration of 1MeTIQ and 1BnTIQ. We have found that 1MeTIQ did not change the level of dopamine and HVA in all investigated structures both after a single and chronic administration. However, the levels of intermediary dopamine metabolites, DOPAC and 3MT, were distinctly affected. The level of DOPAC was strongly depressed (by 60–70%) while the level of extraneuronal matabolite 3MT was significantly elevated (by 170–200%). In contrast to 1MeTIQ, 1BnTIQ depressed the level of dopamine (by approximately 60%) and increased the level of total metabolite, HVA, (by 40%) especially in the striatum, but the levels of DOPAC and 3MT remained unchanged. The paper has shown that 1MeTIQ and 1BnTIQ produced different effects on dopamine catabolism. Potential neuroprotective compound 1MeTIQ did not change the rate of total dopamine catabolism, it strongly inhibited the monoamine oxidase (MAO)‐dependent catabolic pathway and significantly activated the catechol‐O‐methyltransferase (COMT)‐dependent O‐methylation. In contrast 1BnTIQ, a compound with potential neurotoxic activity, produced the significant increase of the rate of dopamine metabolism with strong activation of the oxidative MAO‐dependent catabolic pathway. Interestingly, both compounds produced similar antidopaminergic functional effects: antagonism of apomorphine hyperactivity and induction of muscle rigidity. The results may explain the biochemical basis of the neuroprotective and of the neurotoxic properties endogenous brain tetrahydroisoquinoline derivatives.