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. Afferents expressing channelrhodopsin-2 (ChR2) can be selectively activated by transcutaneous photostimuli applied to behaving mice. Despite care taken to precisely target 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 hind paw of mice while measuring nocifensive withdrawal. Different photostimulus waveforms (pulses and ramps) and response metrics (threshold and latency) were compared in mice expressing ChR2 in all afferents (Advillin-ChR2) or preferentially in nociceptors (NaV1.8-ChR2). CFA-induced inflammation 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 NaV1.7 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 inflammation increases nociceptor excitability without affecting phototransduction, and suggests 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 (NaV) channels showed that nociceptor axons normally rely on NaV1.8 but upregulate NaV1.7 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.