A synergistic neurotrophic response to l-dihydroxyphenylalanine and nerve growth factor.

The catecholamine precursor l-dihydroxyphenylalanine (L-DOPA) is the primary therapeutic intervention for Parkinson's disease. Although short-term exposure (30 min) potentiates dopamine (DA) release by elevating quantal size, longer term exposure to L-DOPA (48 hr) promotes neurite outgrowth from midbrain DA neurons in culture. To characterize long term effects of L-DOPA, we used a pheochromocytoma (PC12) line that extends neurites on exposure to nerve growth factor (NGF). L-DOPA potentiated the outgrowth of processes elicited by NGF. This response did not require conversion of L-DOPA to DA, was not caused by agonist effects at DA receptors, and was not blocked by the tyrosine kinase inhibitor genistein. However, similar results were found after exposure to l-n-acetylcysteine or apomorphine, a DA receptor agonist that produces a quinone metabolite, and seemed to correlate with glutathione synthesis. Long-term process elaboration was blocked by L-buthionine sulfoximine, consistent with mediation by an antioxidant mechanism. L-DOPA potentiation of NGF response was important functionally as seen by increased quantal neurotransmitter release from the L-DOPA/NGF-treated neurite varicosities, which displayed both 2-fold greater quantal size and frequency of quantal release. These results demonstrate potentiation by L-DOPA of morphological and physiological responses to neurotrophic factors as well as synergistic induction of antioxidant pathways. Together with effects on transmitter synthesis, these properties seem to provide a basis for the compound's long term presynaptic potentiation of DA release and therapeutic actions.