Antagonist of transient receptor potential melastatin 2 suppresses mechanical hypersensitivity and activation of microglia induced by infraorbital nerve ligation in male rats

Abstract

Activation of microglia is known to be involved in neuropathic pain. However, the pathway that regulates the microglial activation is not completely understood. Transient receptor potential (TRP) melastatin 2 (TRPM2), which is part of the TRP superfamily, is reportedly expressed on microglia and is suggested to be involved in neuropathic pain. To explore the effect of a TRPM2 antagonist on orofacial neuropathic pain and the relationship between TRPM2 and the activation of microglia, experiments were conducted using male rats that underwent infraorbital nerve (ION) ligation as orofacial neuropathic pain models. TRPM2 expression was detected on microglia in the trigeminal spinal subnucleus caudalis (Vc). The immunoreactivity of TRPM2 in the Vc increased after ION ligation. Mechanical threshold for head-withdrawal response was measured using von Frey filament, and it decreased after ION ligation. When the TRPM2 antagonist was administered to the ION-ligated rats, the low mechanical threshold for head-withdrawal response increased, and the number of phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells in the Vc decreased. The number of CD68-immunoreactive cells in the Vc also decreased after the administration of the TRPM2 antagonist in the ION-ligated rats. These findings suggest that TRPM2 antagonist administration suppresses hypersensitivity to mechanical stimulation induced by ION ligation and microglial activation, and TRPM2 is also involved in microglial activation in orofacial neuropathic pain.

Keywords: CD68; Miconazole; Microglia; Orofacial neuropathic pain; Transient receptor potential melastatin 2; Trigeminal spinal subnucleus caudalis.