Methamphetamine‐induced mitochondrial oxidant stress mediated by monoamine oxidase metabolism of dopamine

Methamphetamine is a potent and highly addictive psychostimulant abused by more than 10 million people in the US (NSDUH). Epidemiological studies suggest that methamphetamine increases the risk for developing Parkinson's disease (PD) by as much as ~3 fold (Callaghan et al., 2012, Drug Alcohol Depend p.35; Curtin et al., 2015 Drug Alcohol Depend p.30; Curtin et al., 2016, Neuropharmacology p.30). In spite of this connection, it is unclear why methamphetamine might promote PD. Mitochondrial oxidant stress in substantia nigra pars compacta (SNc) dopaminergic neurons is widely thought to be a key component of pathogenesis in PD. In these neurons, pacemaking activity is associated with Ca2+ oscillations and elevated somatic mitochondrial stress (Surmeier & Schumacker, 2013, JBC p.10736). However, bath application of methamphetamine reduced pacemaking activity and had no effect on somatic mitochondrial stress. Another potential stressor of SNc dopaminergic neurons is cytosolic dopamine (DA), which can be oxidized to reactive quinones (Hattoria et al., 2009, Parkinsonism Relat Disord p.S35). To evaluate the role of oxidized DA in mediating the effects of methamphetamine, a genetically encoded, redox sensitive fluorescent probe (roGFP) targeted to either the mitochondria or cytosol was monitored with two‐photon laser scanning microscopy in ex vivo slice preparations. Methamphetamine had no effect on cytosolic oxidant stress but did increase mitochondrial oxidant stress in terminals and dendrites. Levodopa had the same effect, confirming DA‐dependence. A potential mediator of DA‐dependent mitochondrial oxidant stress is monoamine oxidase (MAO), which is anchored to the outer mitochondrial membrane. MAO metabolism of DA generates electrons, which could be shuttled to the electron transport chain through intermembrance space proteins with disulfide bonds. Indeed, pharmacological or genetic inhibition of MAO eliminated methamphetamine (or levodopa)‐induced mitochondrial oxidant stress. To probe the mechanisms involved, iPSCs were transfected with the redox sensitive probe targeted to the intermembrane space. Consistent with our hypothesis, levodopa increased mitochondrial stress in the intermembrane space. Moreover, MAO metabolism of DA was able to hyperpolarize the inner mitochondrial membrane consistent with an electron shuttle to Complex IV. To determine if MAO metabolism of DA, and subsequent increased mitochondrial oxidant stress, could have deleterious effects in the SNc, mice were chronically administered methamphetamine for two weeks with pretreatment of rasagiline (FDA‐approved MAO inhibitor) or saline. Two weeks after withdrawal, mice were sacrificed and tyrosine hydroxylase positive (TH+) cells counted in the SNc. Mice treated with methamphetamine alone had a significant loss of SNc TH+ neurons, but those that were pre‐treated with the MAO inhibitor rasagiline did not. Taken together, these results demonstrate that methamphetamine leads to the loss of SNc dopaminergic neurons not by increasing cytosolic quinones but by increasing mitochondrial oxidant stress through a MAO‐dependent mechanism. Moreover, the toxicity of methamphetamine could be dramatically attenuated with an FDA approved drug, rasagiline.