Neuropathic pain is a dreadful and debilitating chronic pain state that occurs following injury to the nervous system. Both peripheral and central mechanisms regarding the pathogenesis of neuropathic pain have been proposed. The central glutamatergic system has been a focus of intense research interest for over a decade and a large body of compelling evidence has emerged that indicates a critical role of this system in the neural mechanisms of neuropathic pain. The involvement of the glutamatergic system in neuropathic pain could result from changes in glutamate uptake as well as from persistent and abnormal activation of glutamate receptors and their related intracellular cascades. To date, much effort has been made to investigate contributions of modulating glutamate receptor activation and the associated intracellular cascades to the mechanisms of neuropathic pain. Little is known about the role of regulating the homeostasis of endogenous ligands of glutamate receptors, primarily glutamate itself, in the pathogenesis of neuropathic pain. It has been well documented that the homeostasis of extracellular glutamate is actively and tightly regulated by neuronal and glial glutamate transporters (GTs), which prevents glutamate-mediated neuronal over excitation and neurotoxicity that may play a critical role in a wide range of neurological disorders including those following nerve injury. Our recent studies have demonstrated that both expression and glutamate uptake activity of spinal GTs are altered following peripheral nerve injury and contribute to the induction and maintenance of neuropathic pain behaviors in rats. In this application, we propose to systematically investigate the role of modulating spinal GTs in the pathogenesis of neuropathic pain utilizing an experimental neuropathic pain model. Our main hypothesis is that peripheral nerve injury would alter the expression and glutamate uptake activity of spinal GTs via distinct regulatory mechanisms mediated through such factors as neurotrophic factors, mitogen-activated protein kinases, arachidonic acids, nitric oxide, and glutamate itself. The GT changes would in turn regulate regional glutamate homeostasis and contribute to the pathogenesis of neuropathic pain. This main hypothesis will be examined by utilizing multidisciplinary methods including behavioral and pharmacological evaluation, immunocytochemistry, Western blot, in situ hybridization, RNase protection assay, and assays of glutamate uptake and regional glutamate concentration, to accomplish three specific aims: (1) to examine changes of spinal GT expression and glutamate uptake activity in a neuropathic pain state; (2) to explore the cellular and molecular mechanisms leading to spinal GT changes in a neuropathic pain state; and (3) to investigate contributions of regulating spinal GTs to the development and maintenance of neuropathic pain. This proposed work would yield novel information regarding the role of regulating glutamate uptake in the pathogenesis of neuropathic pain. Importantly, understanding contributions of regulating glutamate uptake to the mechanisms of neuropathic pain would open a new avenue to developing novel pharmacological tools that could improve the clinical management of often debilitating neuropathic pain syndromes.
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