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Dovepress Low-level light therapy It is also well known that photoreception of light has important effects on biological systems independently of the engagement of visual function. Photoreception within the mammalian retina, including humans, is not restricted to the activity of rod and cone cells but extends to a small number of intrinsically photosensitive retinal ganglion cells. These ganglion cells in the inner layer of the retina serve as photoreceptors that provide information regarding environ- mental irradiance for a variety of nonimage-forming light responses including circadian entrainment and the pupillary reflex.17 These cells use another specialized photoreceptor molecule, melanopsin. Melanopsins absorb light through a retinaldehyde chromophore and drive changes in membrane potential via G protein signaling cascades.18 Interestingly, the pineal gland contains pinopsin, a photoreceptor similar to rhodopsin and melanopsin. It consists of an opsin molecule sensitive to blue light (470 nm) and a retinal chromophore.19 Pinopsin is exclusively expressed in the pineal gland and it is not expressed in the retina or other parts of the brain. It is believed that through pinopsin, environmental light resets the endogenous circadian pacemaker that controls the rhythmic production of melatonin independently of the visual pathway. Molecular biological targets of LLLT Early studies showed that LLLT has remarkable effects on living cells in cell cultures (in vitro effects). For example, LLLT-treated fibroblasts show accelerated metabolism, DNA synthesis, and growth rate.20 These notable positive effects on connective tissue in vitro led to the application of LLLT in the treatment of inflammation and improvement of wound healing in vivo. Since the notable effects of LLLT were first described, a question that has attracted a great deal of attention is what molecule or molecules in cells have the features to make them responsible for the effects of LLLT. As mentioned above, wavelengths in the red to near-infrared range have been found to produce the most significant responses in vivo and are consequently the best characterized in terms of their photobiological response. One of the better characterized photoacceptors in mammalian tissues is hemoglobin. Hemoglobin shows differential light absorption based on its redox state. This property allows its quantification and is the principle of pulse oxymetry and other methods of oxygenation monitoring in clinical practice. However, LLLT has shown beneficial effects in fibroblasts, epithelial cells, HeLa cells, and neurons in culture. Since hemoglobin is exclusively expressed in red blood cells, this suggests that alternate intracellular molecules different from Eye and Brain 2011:3 hemoglobin should be the primary photoacceptors mediating LLLT effects. Besides hemoglobin, the most common pho- toacceptors in the red to near-infrared range are also heme- containing metalloproteins: myoglobin and cytochrome oxidase. Nevertheless, other molecules such as superoxide dismutase, cytochrome c, cytochrome b, nitric oxide syn- thase, catalase, guanylate cyclase, and the cryptochromes have also been shown to have photoacceptor capacities.8 Elucidation of the mechanism of action of LLLT seems puzzling in view of the existence of different molecules with photoacceptor capabilities. This is complicated by the fact that the correspondence of a single molecular target to a particular effective wavelength is not straightforward. For example, flavoproteins such as the reduced nicotinamide adenine dinucleotide (NADH)-dehydrogenase have been identified as photoacceptors in both the violet-to-blue and red to near-infrared spectral region.21 At the same time, terminal oxidases and the endogenous antioxidant enzyme superoxide dismutase also show absorption peaks at high wavelengths (670–680 nm) which represents a clear overlap in the absorption spectrum of different photoacceptors.21,22 This signifies that even when a particular molecule is found to mediate a biological effect for the most part, other possible photoacceptors can also contribute at least in some degree to elicit a particular response. Notably, experiments on the nervous system were the first to provide a clue on the identity of the major photoacceptor mediating the effects of LLLT. Kato et al23 suggested that mitochondria in the bird brain could work as photoreceptors for a photobiological process relating to gonadal growth. Sub- sequent experiments demonstrated that isolated mitochondria are sensitive to irradiation with monochromatic light in the red and near-infrared spectrum. For example, illumination of isolated rat liver mitochondria increased adenosine triphos- phate (ATP) synthesis and oxygen consumption.24 Light also increased the mitochondrial membrane potential (∆Ψ) and proton gradient (∆pH). Light alters mitochondrial optical properties, increases adenosine diphosphate/ATP exchange, ribonucleic acid (RNA) and protein synthesis in mitochon- dria, and increases oxygen consumption.25 These effects of LLLT on mitochondria are observed in a wavelength-specific manner. These data suggested not only that the primary photoacceptor mediating the effects of LLLT is localized to mitochondria, but that molecules that absorb LLLT in cells are probably components of the respiratory chain.25 Further identification of mitochondrial photoacceptors was done by means of action spectra analysis. An action spectrum is the description of a biological response to LLLT submit your manuscript | www.dovepress.com Dovepress 55PDF Image | Low-level light therapy of the eye and brain
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