Neuropathic pain is a highly prevalent and severely distressing condition thought to result from spontaneous activity and sensitization of peripheral nerve fibers, especially nociceptors that signal pain from the periphery. Opioid therapy has been shown to be effective in attenuating neuropathic pain clinically; however their central nervous system effects including sedation, impaired cognitive function and abuse potential limit their usefulness in patient care. Input from sensitized nerve fibers can result in an altered spinal cord processing of thermal and mechanical stimuli (central sensitization). Analgesia from opioid receptor activation on nerve fibers causes a decrease of neuropeptide and neurotransmitter release and inhibition of pain signal propagation to the brain. Previous studies have shown that gene therapy treatment using recombinant herpes simplex-1 (HSV-1) viral vector expressing the mu-opioid receptor (MOR) increases MOR expression on peripheral nerve fibers and enhances heat pain analgesia after opioid treatment in rodents. It is unknown if the analgesia from opioid receptor viral vector therapy is mediated at the peripheral or central terminals of nociceptors. The proposed experiments explore the mechanisms of pain reduction through viral vector therapy. The specific aim outlined in this proposal is: 1) To determine if over-expression of MORs in peripheral nerve fibers enhances opioid sensitivity and decreases primary afferent neuron responses to noxious stimulation post morphine application in neuropathic animals. In these experiments we will inoculate the hindpaw of chronic constriction injury (CCI) mice with an HSV-1 viral vector containing either the construct for either the MOR (SGMOR), the ?-opioid receptor in the anti-sense direction (SGAMOR) or the ?-galactosidase gene (SGZ) as a control. We will implement single fiber skin nerve recordings to investigate the peripheral mechanisms of opiate analgesia after gene therapy. The hypotheses of the proposal are that morphine will 1) greatly decrease responses of noxious stimulation in peripheral nociceptors and 2) reduce spontaneous activity from nociceptors in neuropathic (CCI) animals after MOR viral vector therapy. Opioid drugs that target peripheral nociceptors without affecting the central nervous system may be able to reduce pain and prevent adverse side effects seen with many current opioid therapies. The future long-term goal of this project is to use MOR gene therapy for chronic and neuropathic pain conditions in humans.