Contribution of µ opioid receptor-expressing dorsal horn interneurons to neuropathic pain-like behavior in mice

Intersectional genetics have yielded tremendous advances in our understanding of molecularly-identified subpopulations and circuits within the dorsal horn in neuropathic pain. We tested the hypothesis that spinal µ opioid receptor-expressing interneurons (Oprm1-INs) contribute to behavioral hypersensitivity and neuronal sensitization in the spared nerve injury model in mice.

We coupled the use of Oprm1 ^Cre transgenic reporter mice with whole cell patch clamp electrophysiology in lumbar spinal cord slices to evaluate the neuronal activity of Oprm1-INs in the spared nerve injury (SNI) model of neuropathic pain. We used a chemogenetic approach to activate or inhibit Oprm1-INs, followed by the assessment of behavioral signs of neuropathic pain.

We reveal that SNI yielded a robust neuroplasticity of Oprm1-INs. SNI reduced Oprm1 gene expression in the dorsal horn as well as the responsiveness of Oprm1-INs to the selective µ agonist DAMGO. SNI sensitized Oprm1-INs, as reflected by an increase in their intrinsic excitability (rheobase: Sham 38.62 ± 25.87 pA (n=29); SNI 18.33 ± 10.29 pA (n=29), p=0.0026) and spontaneous synaptic activity (sEPSC frequency in delayed firing neurons: sham 0.81 ± 0.67 Hz (n=14); SNI 1.74 ± 1.68 Hz (n=10), p=0.0466) and light brush-induced co-expression of the immediate early gene product, Fos in lamina I-II (%Fos/tdT+: sham 0.42 ± 0.57% (n=3); SNI 28.26 ± 1.92% (n=3), p=0.0001). Chemogenetic activation of Oprm1-INs produced mechanical hypersensitivity in uninjured mice (Saline: 2.91 ± 1.08g (n=6); CNO 0.65 ± 0.34 (n=6), p=0.0006), while chemogenetic inhibition reduced behavioral signs of mechanical hypersensitivity (Saline: 0.38 ± 0.37 g (n=6); CNO 1.05 ± 0.42 g (n=6), p=0.0052) and cold hypersensitivity (saline 6.89 ± 0.88s (n=5) vs CNO 2.31±0.52s (n=5), p=0.0017).

We conclude that nerve injury sensitizes pronociceptive mu opioid receptor-expressing interneurons in mouse dorsal horn. Non-opioid strategies to inhibit these interneurons might yield new treatments for neuropathic pain.