The clinical orofacial pain syndromes are believed to be frequently related to trigeminal nerve damage as a result of trauma or dental surgery and offer a most difficult challenge to therapy. Trigeminal neuropathic pain occurs more frequently than that at spinal levels, but most animal studies on neuropathic pain describe data from animal models of spinal nerve injuries. Thus, in order to clarify the mechanisms underlying trigeminal neuropathic pain, animal models have to be created specifically for the trigeminal system. Evidence has emerged that the maintenance of neuropathic pain depends on descending facilitation from the rostral ventromedial medulla (RVM), a pivotal structure in descending pain modulation. Recent studies suggest that activated glial cells and released proinflammatory cytokines are intimately involved in spinal sensitization after nerve injury. However, a potential contribution of glial-neuronal interactions in the RVM to the development of neuropathic pain has not been studied. We propose to modify a rat model of chronic constriction injury of the infraorbital nerve (CCI-ION) with a new nocifensive measurement and study novel mechanisms underlying descending facilitation of trigeminal neuropathic pain with an emphasis on glial-neuronal interactions in the RVM. Our major hypotheses are that 1) nerve injury induces neuronal plasticity in the RVM through activation of glial and glutamate receptors and related signaling pathways and 2) glial activation and cytokine release affect or facilitate neuronal plasticity through interactions with neuronal glutamate receptors and play a critical role in the development of neuropathic pain.
Aim 1 will set up a reliable method to measure mechanical hyperalgesia and allodynia in the CCI-ION model and to test the hypothesis that glial cells are activated and proinflammatory cytokines are upregulated in the RVM after nerve injury.
Aim 2 will test the hypothesis that the receptors of these cytokines are expressed in RVM neurons and upregulated in the RVM after CCI-ION.
Aim 3 will test the hypothesis that the inhibition of glial function and blockade of cytokine action in the RVM attenuate allodynia associated with CCI-ION.
Aim 4 will test the hypothesis that glial activation and concomitant cytokine release are involved in neural plasticity in the RVM through interactions with neuronal glutamate receptors, and play a critical role in descending facilitation of neuropathic pain. The studies will advance our understanding of the mechanisms of orofacial neuropathic pain and facilitate the design of new therapeutic approaches.
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