It is possible to beneficially influence the behavior of cells with suitable wavelengths and intensities of laser light, and for some narrow uses this may be roughly analogous to a limited form of small molecule drug development. Light can provoke cells into changing their internal operations, just like small molecules, and no doubt has side-effects, just like small molecules. The open access commentary here makes for interesting reading, though it seems that the marketplace for low level laser light treatments is somewhat ahead of the scientific understanding of the basis for benefits.
Mitochondria play key roles in regulating the ageing process. When their membrane potential and function declines, their production of adenosine triphosphate (ATP) reduces and they can signal cell death. This is particularly marked in the energy demanding central nervous system, where the neurons and glia, undergo some key structural and functional changes during ageing.
Recently, photobiomodulation, the application of red to infrared light on body tissues has been reported to alter the course of aged decline. These wavelengths are absorbed by cytochrome c oxidase, the rate limiting enzyme in mitochondrial respiration, increasing its activity along with mitochondrial membrane potential and ATP production. For the neurons, photobiomodulation improves function, as measured by electroretinograms, in the retina of aged mice, together with reducing cell death in a range of experimental pathologies in the brain.
The precise mechanisms used by photobiomodulation are unclear. Mitochondrial and physiological functions are improved, but increased ATP production alone is unlikely to underpin the physiological improvement, as this is relatively temporary. Hence, there are likely to be cascades of signalling between mitochondria and other structures including the nucleus and endoplasmic reticulum that have a wide ranging impact on metabolism that sustain longer term positive changes. For the neurons, several studies have reported that photobiomodulation activates various transcription factors leading to the expression of stimulatory and protective genes related to beneficial cellular features, for example neurogenesis, synaptogenesis, and an increase in neurotrophic growth factors. For the glial cells, the mechanisms are less clear.
A key issue for consideration at this point is whether the photobiomodulation-induced benefits seen in the animal models of ageing can be translated to humans. One problem would be method of application, given the large size of the human brain. Photobiomodulation has been reported to penetrate 20-30mm through a range of body tissues, from bone to brain. Hence, from a transcranial approach, photobiomodulation would only reach cortical layers of the brain (less than 10mm), but it would penetrate the retina.