Selective neural inhibition via photobiomodulation alleviates behavioral hypersensitivity associated with small sensory fiber activation.

Abstract

Photobiomodulation at higher irradiances has great potential as a pain-alleviating method that selectively inhibits small diameter nerve fibers and corresponding sensory experiences, such as nociception and heat sensation. The longevity and magnitude of these effects as a function of laser irradiation parameters at the nerve was explored. In a rodent chronic pain model (spared nerve injury-SNI), light was applied directly at the sural nerve with four delivery schemes: two irradiance levels (7.64 and 2.55 W/cm2 ) for two durations each, corresponding to either 4.8 or 14.4 J total energy, and the effect on sensory hypersensitivities was evaluated. At emitter irradiances of 7.64 W/cm2 (for 240 s), 2.55 W/cm2 (for 720 s), and 7.64 W/cm2 (for 80 s) the heat hypersensitivity was relieved the day following photobiomodulation (PBM) treatment by 37 ± 8.1% (statistically significant, p < 0.001), 26% ± 6% (p = 0.072), and 28 ± 6.1% (statistically significant, p = 0.032), respectively, and all three treatments reduced the hypersensitivity over the course of the experiment (13 days) at a statistically significant level (mixed-design analysis of variance, p < 0.05). The increases in tissue temperature (5.3 ± 1.0 and 1.3 ± 0.4°C from 33.3°C for the higher and lower power densities, respectively) at the neural target were well below those typically associated with permanent action potential disruption. The data from this study support the use of direct PBM on nerves of interest to reduce sensitivities associated with small-diameter fiber activity.