Our overall goal is to characterize the calcium-dependent interaction between intrinsically generated autonomous spiking and extrinsically generated glutamatergic synaptic transmission in shaping mitochondrial oxidant stress in substantia nigra dopaminergic neurons that are vulnerable in Parkinson s disease. In the last year, we have made excellent progress toward this goal. First, we found that SN dopaminergic neurons have a very low intrinsic buffering capacity for calcium, allowing it to readily diffuse between compartments. Second, sustained antagonism of voltage-dependent L-type calcium channels by systemic administration of drug, significantly lowers mitochondrial oxidant stress. Third, antagonizing glutamatergic NMDA receptors, but not metabotropic glutamate receptors, diminishes oxidant stress in dopaminergic neurons; stimulating NMDA receptors raises stress levels. Fourth, blocking receptors controlling the release of calcium from intracellular stores (IP3 and ryanodine receptors), diminished mitochondrial oxidant stress in pacemaking dopaminergic neurons, suggesting that calcium-induced-calcium-release (CICR) and injection of calcium into mitochondria was a key step in the elevation of oxidant stress; this inference is also consistent with our ability to attenuate stress by blocking the mitochondrial uniporter. We are now developing optical probes that will allow us to directly test this hypothesis. Lastly, we have developed a novel brain slice preparation that allows us to electrically activate pedunculopontine glutamatergic fibers synapsing upon dopaminergic neurons. Unexpectedly, these synapses have a high release probability, contrasting them with glutamatergic subthalamic nucleus synapses.