Is L-dopa an endogenous neurotransmitter?

Since the 1960s, L-3,4-dihydroxyphenylalanine (L-dopa), a precursor of dopamine, has been thought to occur in the cytoplasm of catecholaminergic neurones. L-Dopa is traditionally believed to be an inert amino acid that exerts actions and effectiveness in Parkinson's disease via its conversion to dopamine by L-aromatic amino acid decarboxylase. In contrast to this generally accepted idea, Yoshimi Misu and Yoshio Goshima propose, in this Viewpoint article, that L-dopa itself is an endogenous neurotransmitter or neuromodulator in the CNS. This hypothesis is mainly based on the findings that L-dopa is released in a transmitter-like manner and that exogenously applied levodopa produces some responses.