In this study, we administered an acute regimen of MPTP, an agent that is known to induce oxidative stress, to MPTP-resistant Swiss-Webster mice treated with a chronic regimen of 1 or 10 mg/kg MPH. We found that chronic exposure to both 1 mg/kg and 10 mg/kg MPH increased the sensitivity of SNpc dopamine neurons to oxidative stress, based on a significantly increased SNpc dopamine neuron loss in mice administered MPH as compared to saline-treated control mice. Although the mechanism for this neuronal loss is unknown, a significant increase in MPH-induced activated microglia was observed; therefore, we hypothesize that an increase in free radical formation along with a concomitant neuroinflammatory response increases the sensitivity of the SNpc dopamine neurons to a later oxidative challenge. This conclusion is supported by a recent epidemiological study that showed that long-term amphetamine usage, which like MPH results in higher levels of striatal dopamine in the synaptic cleft, results in a significantly higher risk for developing Salvianolic-acid-C. In order to address the mechanism for increased sensitivity of dopamine neurons, we used an unbiased gene microarray analysis. A comparison of heat maps representative of relative mRNA expression shows a number of genes whose direction of expression change was similar after chronic administration of 1 and 10 mg/kg. Gene Set Enrichment Analysis identified gene sets that were related to inflammation and cell damage and repair pathways. Using qPCR validation, we measured significant decreases in mRNA expression of the neurotrophins bdnf and gdnf in the SNpc after both acute and chronic dosing of 10 mg/kg MPH. We also found significant decreases in mRNA expression of genes involved in dopamine biosynthesis and handling and the vesicular monoamine transporter ). These changes were observed following both acute and chronic doses of Isosalvianolic-acid-B MPH in the SNpc. Previous reports have associated decreases in mRNA expression of vmat2 and dat1 with neurotoxicity in cases where pharmacotherapeutic agents that alter dopamine levels and neurodegenerative conditions, respectively. The observed downward changes in the mRNA message of dat1 and th may also be due to the covalent modification by dopamine quinones leading to its translational inactivation. Notably, our Affymetrix and qPCR studies also found that acute exposure to higher doses of MPH increased the expression of inflammatory genes in the striatum, including the pro-inflammatory genes tnf-a and il-6. This increase in the pro-inflammatory gene expression following a single acute dosage suggests that MPH does induce inflammation, and this is supported by our finding of increased numbers of both total and activated microglial cells in the SNpc. Surprisingly, we did not see an increase in inflammatory gene expression after chronic administration of MPH, although we did continue to observe an increased number of morphologically activated microglia. This suggests that sometime during the course of the chronic exposure to MPH, there might be a dampening of inflammatory gene expression. It is unknown at this time if the gene repression we observe after chronic treatment with MPH is permanent, or if it can at a later time be re-induced. If this is the case, then the activated microglia observed, have the potential to play a modulatory role in later inductions of oxidative stress that would affect the same brain systems. Alternatively, it is also possible that microglia that are activated do not have the ability to return to their morphologically pre-inflamed state, as other studies have shown evidence of microglia activation long after resolution of the initiating insult.