Approximately 10-15% of the US population has chronic widespread pain (CWP);while 20-25% of the population has chronic regional muscle pain. The etiology and pathogenesis of painful musculoskeletal conditions are poorly understood. Most of our knowledge about mechanisms of pain has been obtained from studies using cutaneous pain models. Further, peripheral initiators of muscle pain are virtually unknown, but likely key to the development of chronic pain after muscle insult. Recently, we showed that mechanical hyperalgesia induced by muscle insult do not develop in mice with a null mutation of the acid-sensing ion channel, ASICS. However, it is not clear from these experiments whether the absence of ASICS in the DRG innervating muscle (where insult occurs) or in the skin (where testing occurs) is critical for development of mechanical hyperalgesia, and if ASICS is involved in both the early acute phase as well as the later maintenance phase of muscle-induced hyperalgesia. Since models of cutaneous pain are unaffected in mice without ASICS, the data also suggest that expression of ASICS in sensory dorsal root ganglion (DRG) neurons innervating muscle is unique relative to skin. Therefore, these aims will test the hypotheses that 1) ASIC3 in the muscle is a key factor for full development of cutaneous mechanical hyperalgesia induced by muscle insult, 2) ASIC3 mRNA, protein expression in muscle and/or DRG neurons, and ASIC currents in DRG innervating muscle increases in a time-dependent manner after muscle insult.
The specific aims are designed to determine if site specific (muscle vs. skin) expression of ASICS in knockout mice, downregulation of ASICS in wild-type mice, or pharmacological blockade of ASICs in muscle mediates the mechanical hyperalgesia induced by muscle insult. They will also determine will determine ASICS expression - mRNA, protein, and function - at selected times for two weeks after muscle insult. We will analyze TRPV1 simultaneously as a comparison. It is expected that ASICS in muscle, but not skin, will be important for development of secondary cutaneous mechanical hyperalgesia. We further expect that changes in the expression of ASICS in DRG neurons innervating muscle will correlate with the development and duration of mechanical hyperalgesia after muscle insult. Understanding the mediators and molecules that initiate development of chronic muscle pain is critical to development of new treatment strategies aimed at treating musculoskeletal pain. These studies could lead to peripherally based therapeutic approaches to control pain without undesirable CMS (central nervous system) side effects, including gene therapy.
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