This study will investigate the mechanisms by which dopamine contributes to metabolic stress and dysfunction induced by hypoxia/ischemia in the brain of newborn piglets. It will focus on the striatum, a region richly innervated by the nigrostriatal dopaminergic pathway and which is particularly susceptible to ischemic injury. Metabolic stress will be evaluated by the disturbance of metabolic homeostasis; enhanced expression of the HSP-72 gene, induction of apoptosis, altered forskolin binding (a measure of activated adenylate cyclase), and altered dopamine receptor binding. Our experimental models will utilize hypoxic/ischemic conditions ranging from those causing measurable but fully reversible cellular stress to those leading to significant neuronal death and necrosis. The level and duration of the hypoxic/ischemic insult will be carefully controlled and quantitated by measuring oxygen pressure in the striatum and cortex using oxygen dependent quenching of phosphorescence. Hypotheses to be tested are: 1. Metabolism of dopamine in the brain in vivo is very sensitive to changes in oxygen pressure in the capillary bed of the cortex. 2. In the striatum of newborn piglets, dopamine contributes to development of ischemic/hypoxic disturbance of cellular metabolism and cell function through: a.Overstimulation of the NMDA receptors. b.Generation of free radicals during spontaneous autooxidation of the excess dopamine released during ischemia/hypoxia and during its enzymatic oxidation by monoamine oxidase. c.Causing alterations in the sensitivity of the D1-like and D2-like receptors by modifying the Kd for agonists and/or the number of receptors (Bmax). 3. Expression of the heat shock protein-72 gene and induction of apoptosis, in striatum during hypoxic/ischemic conditions and reoxygenation, is partially mediated by dopamine. 4. Excessive release of dopamine during severe ischemic/hypoxic episodes is at least partly responsible for development of neuronal injury in striatum of newborn piglets. Neuronal cell death, the most often used end point for brain injury, will be measured in our study only for the most severe conditions, since our primary focus is on understanding the factors influencing the metabolic disturbances while they are still reversible. As the mechanism(s) by which dopamine influences the metabolic disturbance are quantitated, it will be possible to devise treatments which block the detrimental and enhance the beneficial effects, minimizing neuronal disturbance and the possibility of neuronal death.
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