Nonaminergic striatal neurons convert exogenous L-dopa to dopamine in parkinsonism

In intact striatum, the enzyme dopa decarboxylase is localized predominantly in dopaminergic nerve terminals. In Parkinson disease, loss of dopaminergic neurons is associated with massive depletion of striatal decarboxylase activity. Nevertheless, efficacy of exogenous L-dopa in parkinsonism is generally believed to result from its enzymatic decarboxylation to dopamine in the corpus striatum. It has previously been suggested that, after degeneration of nigrostriatal pathways, decarboxylation of administered L-dopa may occur mainly at such striatal sites as surviving dopaminergic terminals, serotonergic neurons, or capillaries; but currently available data do not favor these hypotheses. Recent experimental studies indicate that a substantial amount of decarboxylase activity is localized in striatal interneurons or efferent neurons that may not normally synthesize monoamines. We propose that after depletion of dopaminergic terminals, these nonaminergic striatal neurons may contain a large fraction of residual dopa decarboxylase activity and may represent an important locus for conversion of administered dopa to functional dopamine in the parkinsonian corpus striatum.