Defects in energy metabolism have been reported in patients with Parkinson's disease (PD), thus implicating disturbances in energy metabolism as a part of the etiology. Although the presence of an energy impairment in PD is not at present identified as a primary event, it is clear that it is present at a time when dopamine neurons are declining in the PD patient. Thus, it is important to understand how metabolic stress affects DA neurons. This study will examine energy stress induced by inhibition of succinate dehydrogenase with malonate or 3-nitroproprionic acid, on DA neurons focusing on 2 areas of potential cellular mediators of the damage, i.e. glutamate receptor overstimulation and oxidative stress. in vivo (rat) and in vitro (rat mesencephalic cultures) will be used for the studies. It is hypothesized that mild metabolic stress reduces energy status (ATP/ADP), decreases Na+/K+ ATPase activity, degrades the membrane potential, which results in glutamate receptor overstimulation, generation of reactive oxygen species and an ensuing oxidative stress. Studies in aim 1 will use in vitro and in vivo approaches to temporally examine ATP/ADP status, Na+/K+ ATPase inhibition and changes in membrane potential in relationship to toxicity during energy stress induced damage. The sensitivity of DA neurons to excitotoxins and the involvement of glutamate receptors in energy stress induced damage will also be explored. Overstimulation of glutamate receptors is postulated to be a link between mild metabolic stress and generation of an oxidative stress. in vivo and in vitro studies in sub aims of 1 and 2 will temporally examine oxidized/reduced glutathione status, protein thiol loss, protein-glutathione mixed disulfide formation and lipid peroxidative damage to provide evidence of an oxidative component to toxicity due either to excitotoxicity or metabolic inhibition. The ability of spin trap agents to protect vs excitotoxicity or damage to DA neurons due to a metabolic stress will provide additional evidence of oxidative stress. It is further hypothesized that DA neurones as compared with other neuronal populations are more susceptible to a mild impairment of energy metabolism because of the intrinsic oxidative stress placed on them due to DA metabolism and oxidation. The vulnerability o DA neurons will be compared both in vivo and in vitro with the GABA population since this is a major neurotransmitter population found in the striatum and substantia nigra. In sub aims of 1 and 2, we will examine the importance of the antioxidant systems glutathione and catalase in protecting DA or GABA neurons during impaired energy metabolism by manipulating these systems to either inhibit, enhance or deplete them. The role of DA metabolism by MAO or DA oxidation will also be addressed in vivo and in vitro to determine if these events contribute to the relative vulnerability of DA neurons to energy stress. These studies will provide important information regarding the cellular mechanisms mediating damage to DA neurons to energy impairment and the possible reasons for their unique sensitivity and will provide insight into the ongoing loss of the neurons in PD.
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