Selective Optogenetic Inhibition of Slow Conducting Fibers at the Level of the Sciatic Nerve Trunk in the Mouse.

Abstract

Peripheral nerve stimulation is a drug-free alternative for chronic pain management, suppressing nociception through gating mechanisms in the spine. However, excitation through electrical stimulation does not easily discriminate between sensory and motor fibers or their functional subtypes, and can cause off-target effects. Targeted optogenetic inhibition may be a more selective method to suppress nociceptive activity directly while leaving neighboring fibers unaffected.In this study, the chloride-specific light-gated ion pump halorhodopsin (enhanced Natronomonas halorhodopsin [eNpHR]) was introduced into the sciatic nerve of eight adult C57BL/6 mice through intrasciatic injection of adeno-associated virus 6-hSyn-eNpHR-enhanced yellow fluorescent protein (eNpHR-eYFP). After three to four weeks, an acute electrophysiological study was performed to assess optical inhibition of slow conducting electrically evoked compound action potential (eCAP) activity using light directed to the main nerve trunk. Immunohistochemical characterization of eNpHR-eYFP expression was performed in L3-L5 dorsal root ganglion (DRG) neurons.eNpHR-eYFP was detected in the cell bodies and axons of a subset of DRG neurons (∼7%), with soma size and expression pattern consistent with nociceptors (75% of eNpHR-eYFP cells have a soma size of ≤267 μm). The amplitude of slow velocity eCAPs (mean conduction velocity 0.6 m/s) was significantly reduced in the presence of yellow light (∼20% reduction; Wilcoxon signed-rank test, p = 0.008). The amplitude of fast- (22 m/s) and medium- (6 m/s) conducting eCAPs was not affected.This study extends the current understanding of using light-mediated inhibition of nociceptive activity in a mixed peripheral nerve to the level of the axon and measures the underlying changes in neural activity in the sciatic nerve. This study provides evidence for selective light-mediated inhibition at the site of the axon, with potential applications for the suppression of chronic pain.