Models and mechanisms for the transition of acute-to-chronic orofacial pain

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DESCRIPTION (provided by applicant): NIDCR Proposal: Models and mechanisms for the transition of acute-to-chronic orofacial pain Project Summary/Abstract This application addresses broad Challenge Area (15) Translational Science & specific Challenge Topic 15- DE-102*: New Models and Measures in Pre-Clinical Chronic Pain Research. The critical features that predict the transition from acute to chronic pain remain unresolved. The present proposal explores whether microglial "priming" may be of particular importance in explaining the progression from acute to chronic pain. Activation of microglia & astrocytes mediates diverse enhanced pain states. One important aspect of glial functioning that has not been explored in the context of pain is the effect of a sensitized, or "primed", microglial response. Research outside of the field of pain indicates that the past history of microglial activation can greatly alter their response to new challenges. Microglia can reach a primed state via prior stress, pain, trauma & inflammation, & exposure to opioids, which strikingly are known co-morbidities for the transition of acute to chronic pain in the trigeminal system. While in such a primed state, microglia now dramatically over-respond to new challenges, stronger & longer than before. We believe such prior microglial priming can set the stage for the transition of acute to chronic pain in temporomandibular joint (TMJ) disorders & other orofacial pain disorders. Re-activation of primed spinal microglia may lead to a transition from acute pain to chronic pain as a result of a neuroinflammatory response that is greatly amplified in both magnitude & duration. This proposal aims to develop new rat models for the study of the transition from acute to chronic orofacial pain, based on the premise that a first challenge (prior pain, stress, trauma/inflammation, opioids) will markedly enhance pain induced by a subsequent challenge to the trigeminal system (facial allodynia induced by inflammation of either the TMJ or dura). Once robust models are defined & refined, an initial exploration of potential glial cell influence on the transition from acute to chronic pain will be undertaken. This is, by necessity of time constraints, meant as simply the first step toward a thorough investigation to be undertaken in a future proposal based on the data generated by this project. Here, the most robust models will be determined for study using the two blood brain barrier permeable glial activation inhibitors now approved by the FDA for clinical trials aimed at treating neuropathic pain: ibudilast (AV411) & propentofylline (SLC022). These non-opioid, non-addictive drugs will be tested in an initial screen to determine whether either or both compounds may be able to prevent the transition of acute to chronic pain. If they do, as expected, this would suggest that preventing or suppressing glial priming may provide a significant advance in our basic science understanding of how acute pain becomes chronic, as well as provide a clinically testable means by which to prevent & reverse the transition to chronicity. Exploring how known co-morbidities set the stage for the transition from acute to chronic pain by inducing microglia to enter into an over-reactive primed state is a topic never before explored & exciting in its potential practical & theoretical applications. PUBLIC HEALTH RELEVANCE: This proposal aims to develop new rat models for the study of the transition from acute to chronic orofacial pain, based on the premise that a first challenge ("Hit 1": prior pain, stress, trauma/inflammation, opioids) will markedly enhance pain induced by a subsequent challenge to the trigeminal system ("Hit 2": inflammation of either the TMJ or dura). We believe that this transition to chronic pain will be due to sensitization of glia by Hit 1, causing them to massively over respond in response to Hit 2, and that treatment with clinically-relevant glial activation inhibitors will prevent the transition to chronic pain.