Quantifying the contribution of somatosensory afferent types and changes therein to pain sensitivity using transcutaneous optogenetic stimulation in behaving mice

Optogenetics provides an unprecedented opportunity to delineate how different somatosensory afferents contribute to sensation, including pain. By expressing channelrhodopsin-2 (ChR2) in certain afferents, those afferents can be selectively activated by transcutaneous photostimuli applied to behaving mice. Despite the great care taken to precisely target expression of ChR2, imprecise photostimulation has hindered quantitative behavioral testing. Here, using a robot to reproducibly photostimulate behaving mice and precisely measure their paw withdrawal, we show that selectively activating nociceptors with ramped photostimuli evokes faster withdrawal than co-activating nociceptive and non-nociceptive afferents, consistent with gate control. We also show that inflammation-induced hyperexcitability in nociceptors is sufficient to increase pain sensitivity. Electrophysiological testing confirmed that inflammation increases nociceptor excitability without affecting phototransduction. Data further suggest 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 inflammation, 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.