Glutamate and its receptors play an important role in transmission in the spinal cord. There is compelling evidence that glutamate and its iontropic receptors are involved in sensory transduction in the periphery. For example, NMDA, AMPA and kainate (KA) receptors are localized on unmyelinated sensory afferents in rat glabrous skin. Intraplantar injection of glutamate or specific glutamate agonists results in nociceptive behaviors which are blocked by appropriate receptor antagonists. Furthermore, formalin-induced nociceptive behaviors are attenuated by local injection of glutamate antagonists. Finally, glutamate applied to the receptive fields of nociceptors will sensitize these afferents. Thus, multiple lines of evidence support a role for peripheral glutamate receptors in acute pain mechanisms. We now have evidence that peripheral glutamate receptors play a role in persistent inflammatory pain. The central hypothesis which is the focus of this proposal is that peripheral glutamate receptors on sensory afferents and sympathetic efferents are integral components of inflammation-induced pain and that nerve growth factor is pivotal in regulating peripheral glutamate receptor populations and their function during inflammation. This central hypothesis will be explored using a multifaceted approach in rats which have complete Freund s adjuvant-induced inflammation of the hindpaw. Anatomical studies at the electron microscopic level will document the increase in proportions of sensory and sympathetic axons immunostained for NMDA, AMPA or KA receptors in the inflamed hindpaw. Western blot analysis will document increases in glutamate receptor density in peripheral nerves from inflamed animals. In pharmacological studies, the ability of glutamate receptor agonists and antagonists to interfere with inflammation-induced nociceptive behaviors will be assessed. In electrophysiological studies, glutamate-evoked nerve activity in the inflamed hindpaw will be documented. Using microdialysis, the glutamate-evoked release of norepinephrine from sympathetic efferents in inflamed hindpaws will be assessed. The regulation of glutamate receptor expression by nerve growth factor will be documented to elucidate the mechanisms underlying glutamate receptor increases during inflammation. The experiments outlined in this proposal will provide fundamental pre-clinical data necessary to determine the therapeutic potential for management of persistent pain through manipulation of peripheral glutamate and glutamate receptors.
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