The presence of l-3,4-dihydroxyphenylalanine (l-DOPA) immunoreactivity is reported for the first time in some neurons in the human mesencephalic region, using an immunohistochemical method with a newly raised, highly specific anti-l-DOPA antiserum. We have found many l-DOPA-positive/dopamine (DA)-positive and a few l-DOPA-positive/DA-negative cell bodies in dopaminergic regions in the midbrain. The present results suggest the existence of more than one neuronal group of l-DOPA in the human mesencephalon. l-DOPA in one group is an intermediate metabolite for decarboxylation to DA and in another group may exist as an endproduct. l-DOPA in the latter neurons could be a neuromodulator and/or neurotransmitter. Thus, we suggest that l-DOPA plays an important role besides being an intermediate of DA in the human mesencephalon.
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