Sickle cell disease (SCD) is accompanied by acute painful episodes ("crises") superimposed on chronic pain. Opioids are the only therapy for severe pain, but high doses of opioids are required to treat pain in SCD. Chronic opioid use may lead to secondary adverse effects and opioid tolerance. The mechanisms underlying chronic pain in SCD remain unknown. Characterizing pain in SCD and defining the underlying mechanisms is required to develop novel analgesic therapies. Therefore, the goal of this proposal is to use a murine model of SCD to examine peripheral and spinal mechanisms that contribute to pain in this condition. Established mouse models of SCD offer a unique advantage because of their similarity to human genetic, hematologic and pathological disease. We hypothesize that persistent activation of nociceptors by inflammation, hypoxia- reperfusion injury and "crises" leads to central sensitization. Our preliminary data indicate that mice with SCD exhibit cutaneous (mechanical, heat and cold) and deep (decreased grip force) hyperalgesia similar to that observed in patients with SCD. We will employ a multidisciplinary approach with correlative behavioral, electrophysiological and neurochemical studies to examine the following hypotheses: (1) sickle mice exhibit cutaneous and deep hyperalgesia, which can be modulated by peripheral cannabinoids. We will examine cutaneous pain by measuring withdrawal responses to mechanical, heat and cold stimuli and deep/musculoskeletal pain using measurement of grip force. We hypothesize that intraplantar administration of cannabinoid receptor agonists reduce cutaneous hyperalgesia and this occurs through activation of CB1 and CB2 receptors. (2) SCD is accompanied by activation of peripheral nerve fibers and inflammatory mediators of pain in the skin and spinal cord. We will examine the activation of pro-inflammatory cells in the periphery and spinal cord that lead to neuronal activation and sensitization. (3) Pain in SCD involves sensitization of nociceptive spinal neurons. We will record electrophysiological responses of single, identified wide dynamic range and high threshold dorsal horn neurons in control and in SCD mice with hyperalgesia. In the proposed studies, we will use transgenic heterozygous BERK (hBERK) mice expressing sickle hemoglobin and age and sex matched control mice expressing normal human hemoglobin (HbA-BERK). Sickle hBERK mice have a mixed genetic background and carry a single copy of linked transgenes for human 1 and 2S globins. They are homozygous for knockout of murine 1 globin and heterozygous for knockout of murine 2 globin. HbA-BERK controls have an identical genetic background. We expect that sickle mice will show pain characteristics similar to pain in SCD, that activation of CB1 and C2 receptors by cannabinoids will attenuate hyperalgesia and that peripheral and central sensitization contributes to pain in sickle mice. Results of these studies will provide new information on the mechanisms underlying pain in SCD. Understanding the basic mechanisms of pain in SCD will lead to the development of novel and more effective approaches to treat pain in SCD.
Lifelong severe pain impairs quality of life in patients with SCD. High doses of opioids are the only therapy accompanied by secondary side effects. Our goal is to develop an understanding of mechanisms underlying pain in SCD to develop more effective and novel therapies to treat pain in SCD. If our hypotheses prove to be true, our results will provide an understanding that will facilitate treating pain in SCD and in developing novel and more effective analgesics.
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