Continuous determination of 3H-labelled 3,4-dihydroxyphenethylamine (dopamine) synthesis and release in the cat caudate nucleus in vivo.

Three major approaches have been classically used to study the release of dopamine (3,4dihydroxyphenethylamine) from dopaminergic terminals of the nigrostriatal pathway in the cat. The amine has been determined in perfusates (1) from the lateral ventricle (Portig & Vogt, 1969; Von Voigtlander & Moore, 1971), (2) from push-pull cannulae introduced into the head of the caudate nucleus (McLennan, 1964; Riddell & Szerb, 1971), or (3) from a cup placed at the dorsolateral surface of the nuclei (Besson et al., 1971~; ChCramy et al., 1973; Gauchy et al., 1974). The cup technique, which was originally used to observe acetylcholine release from the surface of the cerebral cortex (Mitchell, 1963), was selected for our studies because it enables a large but well-defined part of a structure to be superfused with little damage. In most of our experiments ~-[3,5-~H]tyrosine of high specific radioactivity was introduced continuously into the cup, allowing a sustained formation of 3H-labelled dopamine in the numerous dopaminergic nerve endings of the superfused area. Part of the stored 'H-labelled dopamine 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).