Nonopioid GTS-21 Mitigates Burn Injury Pain in Rats by Decreasing Spinal Cord Inflammatory Responses

Background: Burn injury (BI) pain consists of inflammatory and neuropathic components and activates microglia. Nicotinic alpha 7 acetylcholine receptors (α7AChRs) expressed in microglia exhibit immunomodulatory activity during agonist stimulation. Efficacy of selective α7AChR agonist GTS-21 to mitigate BI pain and spinal pain-mediators was tested. METHODS: Anesthetized rats after hind-paw BI received intraperitoneal GTS-21 or saline daily. Allodynia and hyperalgesia were tested on BI and contralateral paw for 21 days. Another group after BI receiving GTS-21 or saline had lumbar spinal cord segments harvested (day 7 or 14) to quantify spinal inflammatory-pain transducers or microglia activation using fluorescent marker, ionized calcium-binding adaptor protein (Iba1). RESULTS: BI significantly decreased allodynia withdrawal threshold from baseline of ~9–10 to ~0.5–1 g, and hyperalgesia latency from ~16–17 to ~5–6 seconds by day 1. Both doses of GTS-21 (4 or 8 mg/kg) mitigated burn-induced allodynia from ~0.5–1 to ~2–3 g threshold ( P = .089 and P = .010), and hyperalgesia from ~5–6 to 8–9 seconds ( P < .001 and P < .001) by day 1. The GTS-21 group recovered to baseline pain threshold by day 15–17 compared to saline-treated, where the exaggerated nociception persisted beyond 15–17 days. BI significantly ( P < .01) increased spinal cord microgliosis (identified by fluorescent Iba1 staining), microglia activation (evidenced by the increased inflammatory cytokine), and pain-transducer (protein and/or messenger RNA [mRNA]) expression (tumor necrosis factor-α [TNF-α], interleukin-1β [IL-1β], nuclear factor-kappa B [NF-κB], interleukin-6 [IL-6], Janus-associated kinase signal transducer and activator of transcription 3 [JAK-STAT3], and/or N -methyl- d -aspartate receptor [NMDAR]). GTS-21 mitigated pain-transducer changes. The α7AChR antagonist methyllycaconitine nullified the beneficial effects of GTS-21 on both increased nociception and pain-biomarker expression. CONCLUSIONS: Nonopioid, α7AChR agonist GTS-21 elicits antinociceptive effects at least in part by decreased activation spinal-cord pain-inducers. The α7AChR agonist GTS-21 holds promise as potential therapeutic adjunct to decrease BI pain by attenuating both microglia changes and expression of exaggerated pain transducers.