Parkinson's disease (PD), a neurodegenerative disorder of unknown etiology, is characterized by a massive loss of substantia nigra dopaminergic neurons and, to a lesser degree, of locus coeruleus (LC) noradrenergic neurons. Mitochondrial defects have been identified in PD patients, implicating that metabolic stress and impairment of energy metabolism may contribute to the etiology of this disorder. Moreover, disruption of normal DA homeostasis can have adverse effects on DA neurons. The goal of this project is to gain an understanding of the role of the brain vesicular monoamine transporter (VMAT2) and vesicle function in modulating damage to monoaminergic neurons. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its toxic metabolite 1-methyl-4-phenylpyridinium (MPP+) have provided much insight into mechanisms of neurodegeneration of DA neurons. Current thinking is that inhibition of mitochondrial function by MPP+ is a critical component in toxicity whereas MPP+ uptake into vesicles by VMAT2 may provide protection. The first hypothesis to be tested is that the differential vulnerability of dopaminergic neurons in mice and rats as well as the various monoaminergic nuclei within the mouse brain to MPTP/MPP+ is inversely related to the density of VMAT2-containing vesicles. To test this hypothesis, autoradiographic techniques will be used to compare in situ binding of the VMAT2 ligand [3H]-dihydrotetrabenazine within brain regions of mice and rats (Aim 1) and electron microscopy immunocytochemical techniques to analyze vesicle size and number, VMAT2 density/vesicle, and location of these vesicles to somata in the SN and LC of mice (Aim 2). The second hypothesis to be tested is that inhibition of mitochondrial function and homeostasis, increases cytosolic DA concentrations resulting in enhanced oxidative stress and damage. The following studies will be performed to test this hypothesis: 1) source and extent of DA release during the imposed metabolic stress; 2) effects of alterations in VMAT2 function and DA homeostasis prior to or during the metabolic stress on subsequent damage to DA nerve terminals; 3) examination of DA quinone formation as measure of DA oxidation; and 4) examination of vesicle function during metabolic stress (Aims 3 and 4). A comprehensive approach, including both in vivo studies in the striata of mice and rats as well as in vitro studies in cell culture models will be used to investigate the role of DA in the toxicity produced by MPP+ and other metabolic inhibitors. Neuronal degeneration will be assessed using neurochemical measurements. These studies will provide novel and relevant Information as to the contribution of VMAT2-containing vesicles in neuroprotection as well as in neurodegeneration under conditions of a metabolic stress.
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