Recruitment of Aα/Aβ fibers during tonic electrical, optogenetic and combined stimulation in the mouse sciatic nerve

Background Neuromodulation of peripheral nerves via implanted devices produces relief from chronic pain in some patients, but rarely fully eliminates pain. Electrical stimulation lacks selectivity (e.g., between afferent and efferent fibers) to activate only the target Aα/Aβ-fiber axons and must have the stimulating current capped to avoid motor neuron activation, potentially reducing the efficacy of the treatment. Optogenetics offers a potential solution, using genetic tools and light to selectively target the neuromodulation, but responses to tonic optogenetic stimulation decay over time, especially at high frequencies. Combining optogenetic stimulation with electrical stimulation preferentially boosts the response in the targeted population, but it is unknown how responses to combined stimulation are maintained over time. In this study, we measured Aα/Aβ-fiber responses in the sciatic nerve of isoflurane-anaesthetized channelrhodopsin transgenic mice, in response to tonic electrical, optogenetic or combined stimulation delivered at a clinically relevant range of frequencies (4 to 100 Hz; up to 60 seconds). Results Optogenetic stimulation was highly selective for Aα/Aβ-fiber activation with no motor response. When stimulated with light alone, Aα/Aβ-fiber compound action potential (CAP) responses showed an exponential and frequency-dependent ( p < 0.001) reduction in response amplitude, often with complete loss of detectable response at >20 Hz stimulation. Aα/Aβ-fiber CAP responses to electrical stimulation also declined in a frequency-dependent ( p < 0.001), dual-phase exponential manner. The rapid reduction in response size for combined stimulation was greater than that of electrical stimulation ( p < 0.05) before following a similar secondary decline ( p = 0.10). Importantly, responses to combined stimulation were larger than electrical and optical stimulation alone across the stimulation period (>85% of responses were facilitated) and this phenomenon was unaffected by stimulation frequency ( p = 0.33). Conclusion Optogenetic-only methods are untenable for sustained neuromodulation, even at low frequencies. Facilitated responses from combined stimulation allow a more sustained response compared to optical stimulation alone, while still offering the high selectivity of the targeted optogenetic approach. This study provides further evidence for combined stimulation as a potential approach to improve the analgesic effect of existing peripheral nerve stimulation methods.