CTRP9 attenuates peripheral nerve injury-induced mechanical allodynia and thermal hyperalgesia through regulating spinal microglial polarization and neuroinflammation mediated by AdipoR1 in male mice

Peripheral nerve injury triggers rapid microglial activation, promoting M1 polarization within the spinal cord, which exacerbates the progression of neuropathic pain. C1q/TNF-related protein 9 (CTRP9), an adiponectin homolog, is known to suppress macrophage activation and exhibit anti-inflammatory properties through the activation of adiponectin receptor 1 (AdipoR1) in various disease contexts. Nevertheless, the involvement of CTRP9 in microglial polarization in the context of neuropathic pain is still unclear. Our study aimed to how CTRP9 influences spinal microglial polarization, neuroinflammation, and pain hypersensitivity, as well as the underlying mechanism, using a neuropathic pain model in male mice with spared nerve injury (SNI) of sciatic nerve. Our findings revealed SNI elevated the spinal CTRP9 and AdipoR1 levels in microglia. Furthermore, intrathecal administration of recombinant CTRP9 (rCTRP9) substantially weakened mechanical hypersensitivity and heat-related pain response triggered by SNI. On the other hand, rCTRP9 mediated a phenotypic switch in microglia, from the pro-inflammatory M1 state to the anti-inflammatory M2 state, by influencing the spinal AMPK/NF-κB mechanism in SNI mice. Additionally, treatment with AdipoR1 siRNA or an AMPK-specific antagonist both reversed the effects of CTRP9 on the phenotypic switching of spinal microglia and pain hypersensitivity. Collectively, these results indicate that CTRP9 ameliorates mechanical hypersensitivity and heat-related pain response, shifted the balance of microglia towards the anti-inflammatory M2 state, and suppresses neuroinflammatory responses by modulating the AMPK/NF-κB pathway, mediated by AdipoR1 activation, in mice with SNI.