Therapeutic photobiomodulation for methanol-induced retinal toxicity

Abstract

Methanol intoxication produces toxic injury to the retina and optic nerve, resulting in blindness. The toxic metabolite in methanol intoxication is formic acid, a mitochondrial toxin known to inhibit the essential mitochondrial enzyme, cytochrome oxidase. Photobiomodulation by red to near-IR radiation has been demonstrated to enhance mitochondrial activity and promote cell survival in vitro by stimulation of cytochrome oxidase activity. The present studies were undertaken to test the hypothesis that exposure to monochromatic red radiation from light-emitting diode (LED) arrays would protect the retina against the toxic actions of methanol-derived formic acid in a rodent model of methanol toxicity. Using the electroretinogram as a sensitive indicator of retinal function, we demonstrated that three brief (2 min, 24 s) 670-nm LED treatments (4 J/cm(2)), delivered at 5, 25, and 50 h of methanol intoxication, attenuated the retinotoxic effects of methanol-derived formate. Our studies document a significant recovery of rod- and cone-mediated function in LED-treated, methanol-intoxicated rats. We further show that LED treatment protected the retina from the histopathologic changes induced by methanol-derived formate. These findings provide a link between the actions of monochromatic red to near-IR light on mitochondrial oxidative metabolism in vitro and retinoprotection in vivo. They also suggest that photobiomodulation may enhance recovery from retinal injury and other ocular diseases in which mitochondrial dysfunction is postulated to play a role.

Figures

Figure 1

Photobiomodulation does not alter blood formate concentrations in methanol-intoxicated rats. Blood formate concentrations were determined before methanol administration and at 24-h intervals after methanol administration for 72 h. Shown are the mean values ± SEM from six rats in each experimental group. Blood formate concentrations did not differ between the methanol-intoxicated and LED-treated, methanol–intoxicated groups (P > 0.05).

Figure 2

Photobiomodulation improves rod and M-cone ERG response in methanol-intoxicated rats. Rod and M-cone (15 Hz/510 nm) ERG analysis was performed after 72 h of methanol intoxication. Shown are the mean values ± SEM from six rats in the untreated control, methanol-intoxicated, and LED-treated, methanol-intoxicated experimental groups and four rats from the LED control group. ERG responses in methanol-intoxicated and LED-treated, methanol-intoxicated rats were significantly lower than those measured in control rats (*, P < 0.001). ERG responses in LED-treated, methanol-intoxicated rats were significantly greater than those measured in methanol-intoxicated rats (†, P < 0.001).

Figure 3

Photobiomodulation improves UV-cone ERG response in methanol-intoxicated rats. UV-cone (25 Hz/380 nm) ERG analysis was performed after 72 h of methanol intoxication. Shown are the mean values ± SEM from six rats in the control, methanol-intoxicated, and LED-treated, methanol-intoxicated experimental groups and four rats from the LED control group. UV-cone ERG responses were recorded from the same animals in which the rod and M-cone responses were recorded. ERG responses in methanol-intoxicated and LED-treated, methanol-intoxicated rats were significantly lower than those measured in control rats (*, P < 0.001). ERG responses in LED-treated, methanol-intoxicated rats were significantly greater than those measured in methanol-intoxicated rats (†, P < 0.05).

Figure 4

Photobiomodulation protects retinal morphology in methanol-intoxicated rats. Outer retinal morphology in representative untreated control (A), LED control (B), methanol-intoxicated (C), and LED-treated, methanol-intoxicated (D) rats. Sections were taken from the posterior pole of the retina within two disk diameters of the optic nerve in any direction. (Toluidine blue, ×450.) (A) rpe, retinal pigment epithelium; os, photoreceptor outer segments; is, photoreceptor inner segments; onl, outer nuclear layer; opl, outer plexiform layer; ipl, inner plexiform layer. (B) The arrows indicate enlargement and swelling of the photoreceptor inner segments, and the circles indicate the fragmented appearance of photoreceptor nuclei. No histopathologic changes were apparent at the light microscopic level in the LED control or LED-treated, methanol-intoxicated groups.

Figure 5

Photobiomodulation protects photoreceptor ultrastructure in methanol-intoxicated rats. Electron micrographs of the rod inner segment region in representative untreated control (A), LED control (B), methanol-intoxicated (C), and LED-treated, methanol-intoxicated (D) rats. The arrows indicate abnormal mitochondrial morphology in photoreceptor inner segments. Photoreceptor mitochondria from LED control or LED-treated, methanol-intoxicated rats exhibited normal morphology with well-defined cristae. (Magnifications: ×5,000.)