Abstract
Morphine and related compounds are the first line of therapy in the treatment of moderate to severe pain. Over time, individuals taking opioids can develop an increasing sensitivity to noxious stimuli, even evolving into a painful response to previously non-noxious stimuli (opioid-induced hyperalgesia; OIH). The mechanism underlying OIH is not well understood although complex intracellular neural mechanisms, including opioid receptor desensitization and down-regulation, are believed to be major mechanisms underlying OIH. However, OIH may also be associated with changes in gene expression. A growing body of evidence suggests that cellular exposure to mu agonists upregulate chemokines/receptors and recent work from our laboratory implicates chemokine upregulation in a variety of neuropathic pain behaviors. Here we characterized the degree to which chemokines/receptors signaling is increased in primary afferent neurons of the dorsal root ganglion (DRG) following chronic morphine sulfate treatment and correlated these changes with tactile hyperalgesic behavior in rodents. We demonstrate that mRNA expression of the chemokine, stromal-derived factor-1 (SDF1/CXCL12) is upregulated following morphine treatment in sensory neurons of the rat. The release of SDF1 was found to be constitutive when compared with the activity dependent release of the C-C chemokine, monocyte chemoattractant protein-1 (MCP1/CCL2) in a line of F11 neuroblastoma-sensory neuron hybrid cells. We further determined that there is pronounced CXCR4 expression in satellite glial cells and following morphine treatment, increased functional CXCR4 expression in sensory neurons of the DRG. Moreover, intraperitoneal administration of the specific CXCR4 antagonist, AMD3100, completely reversed OIH in the rat. Taken together; the data suggest that opioid-induced SDF1/CXCR4 signaling is central to the development of long lasting OIH and that receptor antagonists represent a promising novel approach to the management of the side effects associated with the use of opioids for chronic pain management.