TRPM3 channels play roles in heat hypersensitivity and spontaneous pain after nerve injury

Transient receptor potential melastatin 3 (TRPM3) is a heat-activated ion channel in primary sensory neurons of the dorsal root ganglia (DRGs). Pharmacological and genetic studies implicated TRPM3 in various pain modalities, but TRPM3 inhibitors were not validated in TRPM3-/- mice. Here we tested two inhibitors of TRPM3 in male and female wild-type and TRPM3-/- mice in nerve injury-induced neuropathic pain. We found that intraperitoneal injection of either isosakuranetin or primidone reduced heat hypersensitivity induced by chronic constriction injury (CCI) of the sciatic nerve in wild-type, but not in TRPM3-/- mice. Primidone was also effective when injected locally in the hindpaw or intrathecally. Consistently, intrathecal injection of the TRPM3 agonist CIM0216 reduced paw withdrawal latency to radiant heat in wild-type, but not in TRPM3-/- mice. Intraperitoneal injection of 2 mg/kg, but not 0.5 mg/kg isosakuranetin, inhibited cold and mechanical hypersensitivity in CCI, both in wild-type and TRPM3-/- mice, indicating a dose-dependent off-target effect. Primidone had no effect on cold sensitivity, and only a marginal effect on mechanical hypersensitivity. Genetic deletion or inhibitors of TRPM3 reduced the increase in the levels of the early genes c-Fos and pERK in the spinal cord and DRGs in CCI mice, suggesting spontaneous activity of the channel. Intraperitoneal isosakuranetin also inhibited spontaneous pain related behavior in CCI in the conditioned place preference assay, and this effect was eliminated in TRPM3-/- mice. Overall, our data indicate a role of TRPM3 in heat hypersensitivity and in spontaneous pain after nerve injury.