Opiate receptor affinities and behavioral effects of enkephalin: Structure-activity relationship of ten synthetic peptide analogues

Synthetic met- and leu-enkephalin bind to rat brain opiate receptors with $\text{1}\text{2}$ and $\text{1}\text{7}$ the affinity of morphine. The aromatic hydroxyl moiety of the tyrosine residue is critical for receptor binding. Intracranial microinjection of met-enkephalin requires very high doses to produce an evanescent, naloxone reversible analgesia and stuperous immobility, presumably because of its rapid enzymatic degradation. Leu-enkephalin fails to elicit analgesia.     

Optogenetic activation of spinal microglia triggers chronic pain in mice

Spinal microglia are highly responsive to peripheral nerve injury and are known to be a key player in pain. However, there has not been direct evidence showing that selective microglial activation in vivo is sufficient to induce chronic pain. Here, we used optogenetic approaches in microglia to address this question employing CX3CR1^creER/+: R26^LSL-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 in both male and female mice. In addition, activation of microglial ReaChR up-regulated neuronal c-Fos expression and enhanced C-fiber responses. Mechanistically, ReaChR activation led to a reactive microglial phenotype with increased interleukin (IL)-1β production, which is likely mediated by inflammasome activation and calcium elevation. IL-1 receptor antagonist (IL-1ra) 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.

This study uses red light activation of channelrhodopsin in spinal microglia to trigger chronic pain hypersensitivity in awake mice, revealing that optogenetic activation of microglia increases IL-1β production via inflammasome activation and calcium elevation, leading to neuronal hyperactivity and chronic pain.