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

Abstract

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.

Modulation of SUR1 KATP Channel Subunit Activity in the Peripheral Nervous System Reduces Mechanical Hyperalgesia after Nerve Injury in Mice

“…To create the neuropathic pain model, spinal nerve ligation (SNL) was performed on adult C57Bl6 mice [37]. The SNL surgery was performed, as previously described, on mice under deep isoflurane anesthesia and was completed within 20 min [38]. Using aseptic technique, an incision over L5-S1 was made, the left L6 transverse process was partially removed and the left L5 nerve was isolated and transected (ipsilateral).…”

Advances in Achieving Opioid Analgesia Without Side Effects

“…There are numerous preclinical studies, in which these PENK- or POMC-encoding vectors were applied intramuscularly, on the spinal cord, intra-articularly, or into the skin/subcutaneous tissue, which resulted in enhanced expression of the respective peptides (Met/Leu-enkephalin or β-endorphin) in the corresponding tissue. Consequently, these treatments led to attenuation of mechanical and heat hypersensitivity in models of inflammatory, neuropathic, or cancer pain, mediated by spinal or peripheral opioid receptors; these analgesic effects were rather modest, but in some cases persisted for several weeks ( Machelska et al, 2009 ; Simonato et al, 2013 ; Goss et al, 2014 ; Hu et al, 2016 ; Klein et al, 2018 ). Since this strategy targets peripheral and spinal cord tissue, the opioid side effects mostly mediated in the brain are not anticipated, but this has not been verified.…”

“…Based on the corresponding pre-clinical studies it is anticipated that HSV-encoding PENK is taken up by cutaneous terminals of peripheral sensory neurons and axonally transported to their cell bodies in DRG, where PENK is processed to enkephalins. The enkephalins can be then transported toward peripheral and central DRG neuron terminals, released and respectively activate peripheral and spinal opioid receptors to provide analgesia ( Antunes Bras et al, 1998 , 2001 ; Goss et al, 2001 ; Klein et al, 2018 ) (Figure 5B ). Similar strategy can also be used to enhance expression of opioid receptors.…”

“…For example, HSV-encoding μ-receptors applied to mouse hind paw elevated μ-receptor-immunoreactivity in epidermal skin fibers, DRG cells, and dorsal horn spinal cord, alleviated basal mechanical hypersensitivity, and enhanced analgesic effects of morphine and peripherally acting loperamide injected systemically in a neuropathic pain model (Table 2 ). Surprisingly and not clarified yet, combined treatment with HSV-encoding μ-receptors and HSV-encoding PENK was ineffective ( Klein et al, 2018 ).…”

Design, synthesis and evaluation of 111In labeled DOTA-conjugated tetrapeptides having high affinity and selectivity for mu opioid receptors

“…Peripheral opioid receptors are therapeutic targets for control of inflammatory [1] and neuropathic [2] pain, as well as relief of opioid-induced bowel dysfunction [3]. Interestingly, the sites also mediate cardiovascular effects, pruritus, urinary retention, wound healing, and infection virulence [4][5][6].…”