Discoveries are rapidly being made in multiple laboratories that shed "light" on the fundamental molecular and
cellular mechanisms underlying the use of low level light therapy (LLLT) in vitro, in animal models and in clinical
practice. Increases in cellular levels of respiration, in cytochrome c oxidase activity, in ATP levels and in cyclic
AMP have been found. Increased expression of reactive oxygen species and release of nitric oxide have also been
shown. In order for these molecular changes to have a major effect on cell behavior, it is likely that various
transcription factors will be activated, possibly via different signal transduction pathways. In this report we compare
and contrast the effects of LLLT in vitro on murine embryonic fibroblasts, primary cortical neurons, cardiomyocytes
and bone-marrow derived dendritic cells. We also examined two human cell lines, HeLa cancer cells and HaCaT
keratinocytes. The effects of 810-nm near-infra-red light delivered at low and high fluences were addressed.
Reactive oxygen species generation, transcription factor activation and ATP increases are reported. The data has led
to the hypothesis that cells with a high level of mitochondrial activity (mitochondrial membrane potential) have a
higher response to light than cells with low mitochondrial activity.