L-dopa-induced dopamine synthesis and oxidative stress in serotonergic cells

L-dopa is a precursor for dopamine synthesis and a mainstay treatment for Parkinson's disease. However, L-dopa therapy is not without side effects that may be attributed to non-dopaminergic mechanisms. Synthesized dopamine can be neurotoxic through its enzymatic degradation by monoamine oxidase (MAO) to form the reactive byproduct, hydrogen peroxide and hydroxyl radicals or through auto-oxidation to form highly reactive quinones that can bind proteins and render them non-functional. Since L-dopa could be decarboxylated by aromatic amino acid decarboxylase (AADC) present within both dopamine and serotonin neurons, it was hypothesized that serotonin neurons convert L-dopa into dopamine to generate excessive reactive oxygen species and quinoproteins that ultimately lead to serotonin neuron death. To examine the effects of L-dopa on serotonin neurons, the RN46A-B14 cell line was used. These immortalized serotonergic cell cultures were terminally differentiated and then incubated with varying concentrations of L-dopa. Results show that RN46A-B14 cells contain AADC and can synthesize dopamine after incubation with L-dopa. Furthermore, L-dopa dose-dependently increased intracellular reactive oxygen species (ROS) and cell death. Dopamine, ROS production and cell death were attenuated by co-incubation with the AADC inhibitor, NSD-1015. The MAO inhibitor, pargyline, also attenuated cell death and ROS after L-dopa treatment. Lastly, quinoprotein formation was enhanced significantly by incubation with L-dopa. Taken together, these data illustrate that serotonergic cells can produce dopamine and that the accumulation of dopamine after L-dopa and its subsequent degradation can lead to ROS production and death of RN46A-B14 serotonergic cells. (C) 2012 Elsevier Ltd. All rights reserved.