Overexpression of µ-Opioid Receptors in Peripheral Afferents, but Not in Combination with Enkephalin, Decreases Neuropathic Pain Behavior and Enhances Opioid Analgesia in Mouse.

BACKGROUND: The current study used recombinant herpes simplex virus type I to increase expression of µ-opiate receptors and the opioid ligand preproenkephalin in peripheral nerve fibers in a mouse model of neuropathic pain. It was predicted that viral vector delivery of a combination of genes encoding the µ-opioid receptor and preproenkephalin would attenuate neuropathic pain and enhance opioid analgesia. The behavioral effects would be paralleled by changes in response properties of primary afferent neurons. METHODS: Recombinant herpes simplex virus type 1 containing cDNA sequences of the µ-opioid receptor, human preproenkephalin, a combination, or Escherichia coli lacZ gene marker (as a control) was used to investigate the role of peripheral opioids in neuropathic pain behaviors. RESULTS: Inoculation with the µ-opioid receptor viral vector (n = 13) reversed mechanical allodynia and thermal hyperalgesia and produced leftward shifts in loperamide (ED50 = 0.6 ± 0.2 mg/kg vs. ED50 = 0.9 ± 0.2 mg/kg for control group, n = 8, means ± SD) and morphine dose-response curves (ED50 = 0.3 ± 0.5 mg/kg vs. ED50 = 1.1 ± 0.1 mg/kg for control group). In µ-opioid receptor viral vector inoculated C-fibers, heat-evoked responses (n = 12) and ongoing spontaneous activity (n = 18) were decreased after morphine application. Inoculation with both µ-opioid receptor and preproenkephalin viral vectors did not alter mechanical and thermal responses. CONCLUSIONS: Increasing primary afferent expression of opioid receptors can decrease neuropathic pain-associated behaviors and increase systemic opioid analgesia through inhibition of peripheral afferent fiber activity.

Enhanced peripheral analgesia using virally mediated gene transfer of the mu-opioid receptor in mice.

BACKGROUND: The use of opioids to treat pain is often limited by side effects mediated through the central nervous system. The current study used a recombinant herpes simplex virus type 1 to increase expression of the mu-opioid receptor (muOR) in primary afferent neurons. The goal of this strategy was to enhance peripheral opioid analgesia. METHODS: Cutaneous inoculation with herpes simplex virus containing muOR complementary DNA (cDNA) in antisense (SGAMOR) or sense (SGMOR) orientation relative to a constitutive promoter, or complementary DNA for Escherichia coli lac Z gene as a control virus (SGZ) was used to modify the levels of muOR in primary afferents. The effects of altered muOR levels on peripheral analgesia were then examined. RESULTS: At 4 weeks after SGAMOR and SGMOR infection, decreased and increased muOR immunoreactivity was observed in ipsilateral dorsal hind paw skin, lumbar dorsal root ganglion cells, and superficial dorsal horns, respectively, compared with SGZ. This change in muOR expression in mice by SGAMOR and SGMOR was accompanied at the behavioral level with a rightward and leftward shift in the loperamide dose-response curve, respectively, compared with SGZ. CONCLUSIONS: This gene therapy approach may provide an innovative strategy to enhance peripheral opioid analgesia for the treatment of pain in humans, thereby minimizing centrally mediated opioid side effects such as sedation and addiction.