Chronic pain resulting from orofacial disorder can be intractable and often involves neuropathic processes. Analgesic drugs used for treating acute pain are not very effective for chronic pain, partly because most analgesic drugs are designed to target neurons. Compared to neurons, the role of glial cells in chronic pain is less defined. Increasing evidence shows that spinal glia (e.g., microglia and astrocytes) are markedly activated following nerve injury and contribute to the development of neuropathic pain, one of the best characterized chronic pain conditions. However, it is largely unknown as to how glia interact with neurons in chronic pain conditions. The overall goal of this application is to investigate how injury-induced neural activity from the periphery results in an activation of spinal microglia, and further, how glial products influence dorsal horn neuron activity. Results from our previous studies have shown that nerve injury induces a drastic activation (phosphorylation) of p38 MAPK (mitogen-activated protein kinase) specifically in spinal microglia, and blockade of this activation suppresses the development of neuropathic pain. Therefore, p38 phosphorylation can be used as a pain-related marker for spinal microglial activation. Our preliminary data show that spinal infusion of the chemokine fractalkine strongly activates p38 in the spinal cord, which provides further evidence that fractalkine is involved in mediating neuronal/glial interaction and neuropathic pain. We propose to produce a nerve injury condition (spinal nerve ligation) to mimic the neuropathic aspect of orofacial pain and test our hypotheses via the following 3 specific aims: (1) To establish that fractalkine, liberated by injury-evoked nerve activity, causes p38 activation, in spinal microglia; (2) To define whether p38 activation induces the synthesis of the proinflammatory cytokines IL-1? and IL-6 in spinal microglia via an upregulation of the transcription factor NF-KB; and (3) To determine whether IL-1?, IL-6, and fractalkine rapidly enhance synaptic transmission and also induce the expression of the pronociceptive genes NK-1 and COX-2 in spinal laminae I-II neurons. We will combine the methods of behavioral testing, whole cell patch clamp recording in spinal slices, immunofluorescence, in situ hybridization, Western blot, and kinase assay to accomplish these aims. Our studies will reveal new mechanisms for neuronal- glial interactions, which may help develop new and more effective therapies for the management of chronic pain including orofacial pain conditions. ? ? ?
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