Optogenetic activation of spinal microglia triggers chronic pain in mice

Abstract

Spinal microglia are highly responsive to peripheral nerve injury and are known to be a key player in neuropathic pain. However, there has not been any direct evidence showing selective microglial activation in vivo is sufficient to induce chronic pain. Here we used optogenetic approaches in microglia to address this question employing CX3CR1creER/+: R26LSL-ReaChR/+ transgenic mice, in which red-activated channelrhodopsin (ReaChR) is inducibly and specifically expressed in microglia. We found that activation of ReaChR by red light in spinal microglia evoked reliable inward currents and membrane depolarization. In vivo optogenetic activation of microglial ReaChR in the spinal cord triggered chronic pain hypersensitivity lasting for 5-7 days. In addition, activation of microglial ReaChR upregulated neuronal c-fos expression and enhanced C-fiber responses. Mechanistically, ReaChR activation led to a reactive microglial phenotype with increased IL-1β production. IL-1 receptor antagonist was able to reverse the pain hypersensitivity and neuronal hyperactivity induced by microglial ReaChR activation. Therefore, our work demonstrates that optogenetic activation of spinal microglia is sufficient to trigger chronic pain phenotypes by increasing neuronal activity via IL-1 signaling.

The voltage-gated proton channel Hv1 promotes microglia-astrocyte communication and neuropathic pain after peripheral nerve injury

“…Progress in the RNA sequencing (RNA-seq) techniques demonstrated a multiple spectrum of reactive microglial subtypes in the diseased conditions [ 61 , 62 ]. Recent RNAseq studies showed that pro-inflammatory cytokine and immunomodulatory drivers: complements, FcɣRs, and INFs are the important upstream mediators of nerve injury-induced gene regulation in reactive microglia in abnormal maladaptive state over long periods of pain hypersensitivity [ 16 ]. It was well documented that microglia are critical in developing neuropathic pain especially in the initiation, during which, the release of BDNF, cytokines such as TNF-α and IL-1β are essential for the establishment of pain hypersensitivities [ 63 , 64 ].…”

“…Using microglia ablation and chemogenetic approaches, studies have shown that microglial proliferation and neuroinflammation in response to nerve injury are key players in the induction of pain hypersensitivities [ 11 – 15 ]. Interestingly, optogenetic activation of spinal microglia is able to directly trigger “microgliogenic pain” [ 16 ]. The most well-known molecular mechanism involves microglial P2X4 receptor (P2X4R), which induces BNDF release and dis-inhibition in spinal lamina I neurons leading to pain hyperalgesia [ 17 – 20 ].…”