Biphasic dose response in low level light therapy - an update

Abstract

Low-level laser (light) therapy (LLLT) has been known since 1967 but still remains controversial due to incomplete understanding of the basic mechanisms and the selection of inappropriate dosimetric parameters that led to negative studies. The biphasic dose-response or Arndt-Schulz curve in LLLT has been shown both in vitro studies and in animal experiments. This review will provide an update to our previous (Huang et al. 2009) coverage of this topic. In vitro mediators of LLLT such as adenosine triphosphate (ATP) and mitochondrial membrane potential show biphasic patterns, while others such as mitochondrial reactive oxygen species show a triphasic dose-response with two distinct peaks. The Janus nature of reactive oxygen species (ROS) that may act as a beneficial signaling molecule at low concentrations and a harmful cytotoxic agent at high concentrations, may partly explain the observed responses in vivo. Transcranial LLLT for traumatic brain injury (TBI) in mice shows a distinct biphasic pattern with peaks in beneficial neurological effects observed when the number of treatments is varied, and when the energy density of an individual treatment is varied. Further understanding of the extent to which biphasic dose responses apply in LLLT will be necessary to optimize clinical treatments.

Keywords: biphasic dose response; low level laser therapy; nitric oxide; photobiomodulation; reactive oxygen species; traumatic brain injury.

Figures

FIG. 1.

Schematic depiction of the cellular signaling pathways triggered by LLLT. After photons are absorbed by chromophores in the mitochondria, respiration and ATP is increased but in addition signaling molecules such as reactive oxygen species (ROS) and nitric oxide (NO) are also produced.

FIG. 2.

One possible theory that can explain the simultaneous increase in respiration an production of nitric oxide is the photodissociation of bound NO that is inhibiting cytochrome c oxidase by displacing oxygen.

FIG. 3.

Three-dimensional model of the Arndt-Schulz curve illustrating how either irradiance or illumination time (fluence) can have biphasic dose response effects in LLLT.

FIG. 4.

Mean grading of oral mucositis (OM) in a hamster cheek pouch model treated with 0.9 J/cm2 of 660-nm laser at two different irradiances (55 mW/cm2 for 16 seconds per point or 155 mW/cm2 for 6 seconds per point). Graph redrawn from data contained in (Lopes, Plapler et al. 2009).

FIG. 5.

Mean wound tensile strength obtained after delivering 5 J/cm2 of 670-nm laser at different power densities (4mW/cm2 applied for 1,250 seconds or 15 mW/cm2 for 333 seconds). Graph redrawn from data contained in (Gal, Mokry et al. 2009).

FIG. 6.

Mean percentage of healing induced in a scratch wounded culture of human fibroblasts using different fluences (constant time, increasing irradiance) of 980-nm laser. Graph redrawn from data contained in (Gal, Mokry et al. 2009).

FIG. 7.

Mean area under the curve of wound area over time in a mouse excisional wound healing model treated with a 7 mW (power density of 4 mW/cm2) HeNe (632.8-nm) laser for times ranging from 249 to 2,290 seconds. Graph redrawn from data contained in (Prabhu, Rao et al. 2010).

FIG. 8.

Mean expression levels of reactive oxygen species (ROS, measured by MitoSox red fluorescence), mitochondrial membrane potential (MMP, measured by red/green fluorescence ration of JC1 dye) and ATP (measured by firefly luciferase assay) in primary mouse cortical neurons treated with various fluences of 810-laser delivered at 25 mW/cm2 over times varying from 1.2 to 1200 seconds.

FIG. 9.

Transcranial laser therapy (36 J/cm2 of 810-nm laser delivered at 50 mW/cm2 (12 minutes illumination time) in a spot of 1-cm diameter centered on the top of the mouse head) was used to treat mice with controlled cortical impact TBI four hours after injury. (A) Significant improvement in neurological severity score continuing for 4 weeks after a single treatment. (B) Delivering ten times more light by increasing irradiance tenfold (500 mW/cm2) loses all therapeutic benefit, and produces worse performance soon after laser. (C) Repeating beneficial laser treatment daily for 14 days loses benefit in performance after 5 days.