Changes in Neuronal Dopamine Homeostasis Following 1-Methyl-4-Phenylpyridinium (MPP+) Exposure [Metabolism]

January 16th, 2015 by Choi, S. J., Panhelainen, A., Schmitz, Y., Larsen, K. E., Kanter, E., Wu, M., Sulzer, D., Mosharov, E. V.

MPP+, the active metabolite of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, selectively kills dopaminergic neurons in vivo and in vitro via a variety of toxic mechanisms, including mitochondrial dysfunction, generation of peroxynitrite, induction of apoptosis, and oxidative stress due to disruption of vesicular dopamine storage. To investigate the effects of acute MPP+ exposure on neuronal dopamine homeostasis, we measured stimulation-dependent dopamine release and non-exocytotic dopamine efflux from mouse striatal slices, and extracellular, intracellular and cytosolic dopamine levels in cultured mouse ventral midbrain neurons. In acute striatal slices, MPP+ exposure gradually decreased stimulation-dependent dopamine release, followed by massive dopamine efflux that was dependent on MPP+ concentration, temperature and dopamine uptake transporter activity. Similarly, in mouse midbrain neuronal cultures, MPP+ depleted vesicular dopamine storage accompanied by an elevation of cytosolic and extracellular dopamine levels. In neuronal cell bodies, increased cytosolic dopamine was not due to transmitter leakage from synaptic vesicles but rather due to competitive MPP+ dependent inhibition of monoamine oxidase activity. Accordingly, monoamine oxidase blockers pargyline and L-deprenyl had no effect on cytosolic dopamine levels in MPP+ treated cells and produced only a moderate effect on the survival of dopaminergic neurons treated with the toxin. In contrast, depletion of intracellular dopamine by blocking neurotransmitter synthesis resulted in ~30% reduction of MPP+-mediated toxicity, whereas overexpression of vesicular monoamine transporter 2 completely rescued dopamine neurons. These results demonstrate the utility of comprehensive analysis of dopamine metabolism using various electrochemical methods and reveal the complexity of the effects of MPP+ on neuronal dopamine homeostasis and neurotoxicity.