Chronic pain‐mediated Regulator of G protein signaling 4 (RGS4) gene expression in superficial dorsal horn of spinal cord

Abstract

Regulator of G protein signaling 4 (RGS4) is a potent negative regulator of G-protein coupled receptor signaling duration. Recent studies have identified RGS4 as a key gene in the maintenance of mechanical and cold allodynia associated with chronic pain states in mice. RGS4 is abundantly expressed across the pain matrix, and several studies have demonstrated upregulation of RGS4 mRNA in response to both peripheral inflammation and nerve injury. Here, we tested the hypothesis that peripheral nerve injury alters RGS4 neuronal expression broadly, and with specific expression-patterning among excitatory and inhibitory interneuron populations within the superficial dorsal horn of the mouse spinal cord. Chronic neuropathic pain was induced in C57BL/6 male mice via spared-nerve injury (SNI). At 2.5 months post-surgery (SNI vs sham), animals were sacrificed and lumbar spinal cords were harvested for tissue processing. RNAScope, an ultrasensitive RNA ISH method, was used to visualize and quantify single-molecule expression of RGS4, prodynorphin/PDYN (exclusively expressed by inhibitory dorsal horn interneurons), and somatostatin/SST (expressed by excitatory interneurons) within the cellular topography of the lumbar superficial dorsal horn. RGS4 expression increased significantly in the dorsal horn following spared-nerve injury (P =0.0384). RGS4 mRNA remained unchanged (SNI vs sham) among expression-positive dorsal horn neurons (p=0.6327). In sham-treated mice, there were no differences in cell-type specific (PDYN vs. SST) RGS4 expression patterns in the dorsal horn (p=0.4170). Cell-type specific RGS4 expression in the dorsal horn was significantly different (F (2,27) = 8.353; P =0.0015; one-way ANOVA) following spared-nerve injury. SNI-induced RGS4 expression was significantly enriched in PDYN vs SST-expressing interneurons (p=0.0300), and overall RGS4 expression was significantly downregulated in SST-expressing cells compared to all cells (p=0.0012). These data demonstrate that nerve injury induces upregulation of RGS4 in the superficial dorsal horn, and suggest that RGS4-positive inhibitory interneurons may be implicated in influencing persistent pain signaling within the dorsal horn of the spinal cord.