This project will focus on the role of reactive metabolites of dopamine (DA) in the process of neurodegeneration. We hypothesize that (a) the oxidation of DA to free radicals and reactive DA quinones results in the selective destruction of DA terminals, (b) this will occur with both exogenous and endogenous DA, and ' the selectivity is due to the presence of high levels of cytoplasmic DA within the terminal where oxidation occurs. We have shown in rats that an intrastriatal injection of DA results in the formation of DA oxidation products, proteins-bound cysteinyl- catechols, and show evidence of selective toxicity to DA terminals. In the proposed experiments, we will examine more thoroughly the phenomenon of DA-induced toxicity in the rat model using biochemical indices of DA oxidation and biochemical and immunohistochemical indices of toxicity. First, we will rigorously examine whether exogenous DA produces a selective loss DA terminals, and whether the loss of terminals results in the subsequent loss of DA cells in the substantia nigra. Secondly, we will examine the vulnerability of DA terminals to DA-induced toxicity under conditions more readily associated with the disease such as in the aged, antioxidant-deficient, or metabolically- impaired rat. In addition, we will examine the neurotoxic effects of chronically administered intrastriatal DA and induces of DA oxidation following increased availability of endogenous DA in the 6-OHDA-lesioned rat exposed to L-dOPA. Thirdly, we will examine the role of DA uptake and DA metabolism as the mechanism associated with the selective vulnerability of DA terminals to DA-induced oxidative stress. This group of studies will examine the role of DA as opposed to other oxidants, and the effect of blocking DA uptake or DA metabolism on the selective toxicity to DA terminals. Finally, we will begin to examine potential protein targets of DA quinone binding both intracellularly and extracellularly. Specific effects of extracellular reactive metabolites on DA receptor binding will be examined as part of this study. The results of these studies will help to clarify whether reactive metabolites of DA may be contributing to the pathogenesis associated with the loss of DA neurons in Parkinson's disease as well as identify potentially useful therapeutic interventions.