Pathways of dopamine metabolism in the rabbit caudate nucleus in vitro

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Abstract
Slices of rabbit caudate nucleus were preincubated with 3H-dopamine and then superfused. 3H-dopamine and its metabolites were separated by column chromatography. The basal outflow of tritium consisted of 68% DOPAC, 21% OMDA metabolites (most of which was HVA), 6% dopamine and 5% MTA. Except for an increase in 3H-dopamine, the basal outflow was little changed by nomifensine or cocaine. Amezinium reduced the outflow of 3H-DOPAC and 3H-OMDA metabolites and increased that of 3H-dopamine and 3H-MTA; its effect was antagonized by nomifensine. Haloperidol 10−7 M selectively enhanced the outflow of 3H-DOPAC and 3H-OMDA metabolites. At haloperidol 10−5M, a large increase in these two fractions was accompanied by a small increase in 3H-dopamine and 3H-MTA. Nomifensine diminished only slightly the outflow of 3H-DOPAC caused by haloperidol. The overflow of tritium elicited by electrical stimulation at 3 Hz consisted of 74% DOPAC, 15% OMDA metabolites, 10% dopamine and 0.4% MTA. Lowering the frequency reduced the overflow of total tritium and caused a decrease in the percentage of 3H-DOPAC and an increase in the percentage of 3H-dopamine. Nomifensine or cocaine greatly diminished the evoked overflow of 3H-DOPAC and 3H-OMDA compounds and increased the evoked overflow of 3H-dopamine and 3H-MTA. The effects of apomorphine and bromocriptine were similar to those caused by decreasing the stimulation frequency. The major pathways of the metabolism of previously taken up dopamine, under the conditions of our experiments, are as follows. When the neurones are at rest, dopamine metabolism is initiated by leakage of the amine into the axoplasm, where it is degraded through the aldehyde to DOPAC. Part of the DOPAC is methylated extraneuronally to HVA. Traces of MTA are formed by extraneuronal methylation of dopamine. When action potentials arrive, dopamine metabolism is initiated by exocytosis. The bulk of the extracellular dopamine is taken up back into the neurones. When the stimulation frequency is 3 Hz, most of the axoplasmic dopamine is subsequently transformed to DOPAC; little seems to be re-stored. HVA and MTA are generated essentially as during neuronal rest. When dopamine release is low (stimulation at low frequency; addition of apomorphine or bromocriptine), a larger portion seems to be re-stored, thus leading to a decrease in the percentage of DOPAC. Haloperidol, apart from its receptor blocking properties, acts on dopaminergic axons in a manner akin to the effect of reserpine.