title: L-dopa-induced dopamine synthesis and oxidative stress in serotonergic cells.
abstract: 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.
authors.affiliation: Department of Neurosciences, University of Toledo, College of Medicine, 3000 Arlington Avenue, Mail Stop 1007, Toledo, OH 43614, USA.
authors.forename: Branden J
authors.initials: BJ
authors.lastname: Stansley
authors.forename: Bryan K
authors.initials: BK
authors.lastname: Yamamoto
mesh_terms: D002461:Cell Line, Transformed; D004298:Dopamine / Q000096:biosynthesis*; D006801:Humans; D007980:Levodopa / Q000494:pharmacology*; D018384:Oxidative Stress / Q000187:drug effects / Q000502:physiology*; D017382:Reactive Oxygen Species / Q000378:metabolism; D059326:Serotonergic Neurons / Q000187:drug effects* / Q000378:metabolism*
chemical_list: D017382:Reactive Oxygen Species; D007980:Levodopa; D004298:Dopamine