Current methods to treat pain have several limitations (e.g., addiction, limited efficacy, etc.) and new options are sorely needed. Photobiomodulation (PBM) at 808 nm has been shown to reduce small fiber axon sensitivities in both human and animal models when applied for the purposes of action potential block. A study using a pulsed light dosing scheme was conducted to determine the viability of using commercially available IPGs for a potential implanted system. Results showed a reduction in pain lasting about 8 days, indicating that this method of delivery has promise for an implanted system.
Previous studies examining the focal application of photobiomodulation (PBM) on nerves have consistently yielded reductions in small-diameter fiber activity lasting multiple days. Experiments demonstrating that PBM applied to the sciatic nerve following selective small-diameter fiber excitation would provide further evidence that PBM reduces sensitivity of small-diameter fibers. To investigate this, focal PBM was applied to the rat sciatic nerve, a capsaicin solution was injected into the hind paw, and behavioral data were collected following PBM application. Small fiber heat sensitivities were reduced by over 20% for days following treatment, but mechanical sensitivities were only reduced for one day. To understand translational opportunities, we measured additional effects: focal PBM’s effect on tissue temperature, and PBM’s effect on motor fiber output of the gastrocnemius. Focal PBM application to the sciatic nerve caused an average temperature change of 5oC, and the force generation of the gastrocnemius did not vary following PBM application. Taken together, these data support focal PBM’s translational appeal as a non-addictive, prolonged, pain-reducing therapy.
Previous studies have shown a decrease in sensitivity to noxious thermal stimuli to the limb following focal photobiomodulation (PBM) of the sural nerve. The mechanism by which PBM blocks nociceptive transmission in the sural nerve is unknown. We tested two possible theories for neural inhibition using computational models (NEURON simulation environment). First, the beading phenomenon known as varicosities observed with PBM and second, the observations that nitric oxide or other reactive oxygen species are increased following PBM. Other groups have shown that nitric oxide can block unmyelinated and demyelinated axons by blocking sodium current. We hypothesize that the size increase observed with PBM in vivo is not sufficient to block action potentials, and the results indicate that increasing the axon diameter to block action potentials requires diameter increases much larger than those observed in vivo, suggesting varicosities cannot be solely responsible for the block phenomenon. However, the role of nitric oxide on sodium current is expected to reduce the action potential magnitude enough to cause the block effect, and this mechanism of block merits further investigation.
Previous meta-analyses of photobiomodulation (PBM) parameters have reported correlations between effect size and emitter parameters, but the trends are not robust enough to prospectively predict doses that will yield a significant effect. A meta-analysis of PBM dosing protocols on peripheral nerves with 800 – 840 nm wavelength light was performed using Monte Carlo photon propagation models to elucidate clearer trends between dose and effect. We then tested various doses of PBM applied directly at exposed hindlimb nerve in a rodent pain model. A single application of PBM at the nerve reduced pain associated with heat-sensitive fibers for approximately 7 days.
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