TRPA1 Agonist-Responsive Afferents Contribute to Central Sensitization by Suppressing Spinal GABAergic Interneurons Through Somatostatin 2A Receptors

Abstract

Altered nociception, a key feature of nociplastic pain, often involves central sensitization. We previously found that central sensitization underlying a nociplastic pain state in female mice depends on the ongoing activity of TRPA1 agonist-responsive afferents. Here, we investigated how the activity of these afferents induces and maintains central sensitization at the spinal level. We hypothesized that, in the superficial dorsal horn where somatostatin (SST) is expressed in excitatory interneurons and the SST2A receptor (SST2A-R) in GABAergic inhibitory interneurons (GABAn), TRPA1 agonist-responsive afferents stimulate SST-expressing excitatory interneurons (SSTn), leading to GABAn suppression through SST2A-R and resulting in altered nociception. We tested this hypothesis using ex vivo Ca²⁺ imaging of dorsal root-attached spinal cord slices expressing GCaMP6f in either SSTn or GABAn and in vivo assessment of mechanical hypersensitivity. The dorsal root was chemically (with allyl isothiocyanate [AITC]) and electrically stimulated to activate TRPA1-expressing nociceptors and all afferents, respectively. The stimulation of dorsal root with AITC excited SSTn. During activation of AITC-responsive afferents, a subset of SSTn showed potentiated responses to both low- and high-threshold afferent inputs, whereas a subset of GABAn showed suppressed responses to those afferents in an SST2A-R-dependent manner. Intrathecally administered SST2A-R antagonist inhibited the development of mechanical hypersensitivity by intraplantar AITC injection and alleviated persistent mechanical hypersensitivity in the murine model of nociplastic pain. These results suggest that the activity of TRPA1 agonist-responsive afferents induces and maintains central sensitization by activating dorsal horn SSTn and suppressing GABAn via SST2A-R, resulting in altered nociception that manifests as mechanical hypersensitivity. PERSPECTIVE: This article presents experimental evidence that TRPA1 agonist-responsive afferents induce and maintain central sensitization at the spinal level by activating SST-expressing excitatory interneurons and suppressing GABAergic inhibitory interneurons via SST2A-R. Spinal SST2A-R may represent a promising target for treating mechanical pain hypersensitivity due to central sensitization by TRPA1 agonist-responsive afferents.

Keywords: Somatostatin, GABAergic neuron, SST2A receptor, nociplastic pain, central sensitization.