Continuous Determination of 3H-Labelled 3,4-Dihydroxyphenethylamine (Dopamine) Synthesis and Release in the Cat Caudate Nucleus in vivo

amine is released spontaneously and can be easily determined in superfusates after its separation from labelled tyrosine and metabolites. Simultaneously, information about the rate of conversion of ~-[3,5-~H]tyrosine into 'H-labelled dopa (3,4-dihydroxyphenylalanine) can be obtained continuously and by measuring 'HzO in superfusates. Indeed one molecule of 'HzO is produced for each molecule of 'H-labelled dopa synthesized, and large amounts of labelled water diffuse into superfusates. The measurement of jHzO is a specific index of the rate of tyrosine hydroxylation; this is substantiated by the fact that experiments carried out in rats indicated that the formation of jHzO could no longer be detected following the inhibition of the amine synthesis with a-methylparatyrosine or the complete degeneration of the nigrostriatal dopaminergic pathway induced by injection of 6-hydroxydopamine into the substantia nigra (Besson et al., 19716; Javoy et al., 1974).