Peripheral delta opioid receptors mediate duloxetine antiallodynic effect in a mouse model of neuropathic pain

Abstract

Peripheral delta opioid (DOP) receptors are essential for the antiallodynic effect of the tricyclic antidepressant nortriptyline. However, the population of DOP-expressing cells affected in neuropathic conditions or underlying the antiallodynic activity of antidepressants remains unknown. Using a mouse line in which DOP receptors were selectively ablated in cells expressing Nav1.8 sodium channels (DOP cKO), we established that these DOP peripheral receptors were mandatory for duloxetine to alleviate mechanical allodynia in a neuropathic pain model based on sciatic nerve cuffing. We then examined the impact of nerve cuffing and duloxetine treatment on DOP-positive populations using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP). Eight weeks postsurgery, we observed a reduced proportion of DOPeGFP-positive small peptidergic sensory neurons (calcitonin gene-related peptide (CGRP) positive) in dorsal root ganglia and a lower density of DOPeGFP-positive free nerve endings in the skin. These changes were not present in nerve-injured mice chronically treated with oral duloxetine. In addition, increased DOPeGFP translocation to the plasma membrane was observed in neuropathic conditions but not in duloxetine-treated neuropathic mice, which may represent an additional level of control of the neuronal activity by DOP receptors. Our results therefore established a parallel between changes in the expression profile of peripheral DOP receptors and mechanical allodynia induced by sciatic nerve cuffing.

Peripheral Delta Opioid Receptors Mediate Formoterol Anti-allodynic Effect in a Mouse Model of Neuropathic Pain

“…Our data also revealed that changes in the expression profile of peripheral DOP receptors correlated with mechanical allodynia. Indeed, we observed decreased DOP receptor expression in unmyelinated calcitonin gene-related peptide (CGRP) positive neurons and free nerve endings in the skin and increased surface expression in the neurons still expressing the receptor in neuropathic conditions but not in mice chronically treated with duloxetine after cuff surgery (Ceredig et al, 2018). Here, we sought to determine whether the same mechanisms are triggered by chronic treatment with the β2 adrenergic agonist formoterol by identifying changes in the expression of peripheral DOP receptor using a mouse line in which peripheral DOP receptors are selectively ablated in neurons expressing the Nav1.8 sodium channel (DOP cKO; Gaveriaux-Ruff et al, 2011) and a knock-in mouse line expressing DOP receptors fused to the green fluorescent protein eGFP (DOPeGFP;Scherrer et al, 2006).…”

“…Sham and Cuff groups both received control saccharin solution 0.2% in drinking water. Sham and Cuff groups were identical to those published previously in Ceredig et al (2018).…”

“…Immunohistochemistry was performed according to standard protocols as previously described in Ceredig et al (2018). Briefly, DRG sections were incubated for 1 h at room temperature (RT) in the blocking solution consisting of PB with 0.2% Tween 20 (PBT; Cat.…”

“…Plantar skin of both hind paws (footpad and glabrous skin, 1 cm long) were fixed at 4 • C in 4% PFA solution overnight, cryoprotected overnight with 30% sucrose in PB, embedded in OCT, frozen and kept at −80 • C. Longitudinal cross-sections (50 µm thickness) were cut with a cryostat (MicromCryo-star HM560) and kept floating in PB. Paw tissue samples were then processed to visualize primary afferent terminals in the skin of the hind paw as previously described in Ceredig et al (2018). Briefly, sections were treated with 0.3% H 2 O 2 , dehydrated with successive baths in ethanol then rehydrated, washed 3 times with PBS and incubated in blocking solution (PBS, 0.5% Triton X100 (PBST) with 3% normal goat serum or normal donkey serum) for 30 min at RT.…”

“…The mechanisms underlying the relief of mechanical allodynia are the topic of extensive research in various preclinical models but remain unclear. Interestingly, delta-opioid (DOP) receptors are essential for the antiallodynic effect of DOP agonists (Nozaki et al, 2012;Vicario et al, 2016) but also for the antiallodynic effect of chronic administration of both antidepressants (Benbouzid et al, 2008b;Yalcin et al, 2010;Ceredig et al, 2018;Kremer et al, 2018) and β2 mimetics (Yalcin et al, 2010;Choucair-Jaafar et al, 2014). More specifically, our previous work using the cuff model pointed to peripheral DOP receptors expressed in Nav1.8 positive neurons as mandatory for the antiallodynic action of DOP agonists (Gaveriaux-Ruff et al, 2011;Nozaki et al, 2012) as well as the SNRI duloxetine (Ceredig et al, 2018).…”