Quantifying Changes in Pain Sensitivity Using Reproducible Transcutaneous Optogenetic Stimulation in Behaving Mice

Abstract

Optogenetics provides an unprecedented opportunity to delineate how different somatosensory afferents contribute to sensation, including pain. Afferents expressing channelrhodopsin-2 (ChR2) can be selectively activated by transcutaneous photostimuli applied to behaving mice. Despite targeting expression of ChR2 to specific cell types, imprecise photostimulation has hindered quantitative optogenetic-based behavioral testing. Here, we used a robot to reproducibly apply transcutaneous optogenetic stimuli to the hindpaw of mice while measuring nocifensive withdrawal. Different photostimulus waveforms (pulses and ramps) and response metrics (threshold and latency) were compared in mice of either sex expressing ChR2 in all afferents (Advillin-ChR2) or preferentially in nociceptors (Na V 1.8-ChR2). Inflammation induced by complete Freund’s adjuvant (CFA) caused withdrawal from ramped photostimuli to become faster relative to baseline and to vehicle-injected controls whereas pulsed photostimuli revealed a modest increase in threshold. Analgesia caused by Na V 1.7 and 1.8 channel blockade was evident in both testing protocols. Overall, ramp-based testing was more effective and more efficient (i.e., required less time and total stimulation) than pulse-based testing. Electrophysiological measurements confirmed that CFA increases nociceptor excitability without affecting phototransduction, suggesting that withdrawal latency depends on the number of nociceptors activated rather than how strongly each nociceptor is activated. Consistent with changes described in nociceptor somata, the behavioral consequences of peripherally blocking different voltage-gated sodium (Na V ) channels showed that nociceptor axons normally rely on Na V 1.8 but upregulate Na V 1.7 after CFA, with important clinical implications for drug efficacy. Collectively, these results demonstrate the utility of optogenetic pain testing when reproducibly delivered and strategically designed photostimuli are used.