Mechanical allodynia is pain caused by normally innocuous mechanical stimuli and is a cardinal and intractable symptom of neuropathic pain. Roles of low-threshold mechanoreceptors (LTMRs), including Aβ fibers, in mechanical allodynia have previously been proposed, but the necessity and sufficiency of LTMRs in allodynia have not been fully determined. Recent technological advances have made it possible to achieve subpopulation-specific ablation, silencing or stimulation, and to dissect and elucidate complex neuronal circuitry. Recent studies using an optogenetic approach have shown that activation of LTMRs, including Aβ fibers that genetically express channelrhodopsin-2, by illuminating blue light to the skin elicit morphine-resistant withdrawal behaviors after nerve damage. Whole-cell recording has revealed that optical Aβ stimulation after nerve injury causes excitation of lamina I dorsal horn neurons, which are normally silent by this stimulation. Moreover, Aβ stimulation after nerve injury results in activation of central amygdaloid neurons and produces aversive behaviors. In summary, these findings indicate that optogenetics is a powerful approach for investigating LTMR-derived pain (resembling mechanical allodynia) with sensory and emotional features after nerve injury and for discovering novel and effective drugs to treat neuropathic pain.
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