Abstract
Photobiomodulation (PBM) is a simple, efficient and cost-effective treatment for both acute and chronic pain. We previously showed that PBM applied to the mouse head inhibited nociception in the foot. Nevertheless, the optimum parameters, location for irradiation, duration of the effect, and the mechanisms of action remain unclear.
In the present study, the pain threshold in the right hind-paw of mice was studied, after PBM (810 nm CW laser, spot size 1 cm or 6 cm , 1.2–36 J/cm ) applied to various anatomical locations. The pain threshold, measured with von Frey filaments, was increased more than 3-fold by PBM to the lower back (dorsal root ganglion, DRG), as well as to other neural structures along the pathway such as the head, neck and ipsilateral (right) paw.
On the other hand, application of PBM to the contralateral (left) paw, abdomen and tail had no effect. The optimal effect occurred 2–3 hours post-PBM and disappeared by 24 hours. Seven daily irradiations showed no development of tolerance.
Type 1 metabotropic glutamate receptors decreased, and prostatic acid phosphatase and tubulinpositive varicosities were increased as shown by immunofluorescence of DRG samples. These findings elucidate the mechanisms of PBM for pain and provide insights for clinical practice.
1. INTRODUCTION
Throughout the world, injuries, diseases and a variety of chronic or degenerative conditions affect billions of people with acute and/or chronic pain [1]. Better therapies are required for painful conditions such as degenerative orthopedic conditions, neuropathic pain, and chronic central pain. In general, pharmaceutical therapies (painkillers) decrease the intensity and severity of the symptoms but have many undesired side-effects [2]. Moreover, long-term treatment tends to decrease effectiveness as the subject develops drug tolerance [3].
There has recently been an epidemic of opiate overdose-induced deaths, many of which were in individuals originally prescribed opiates for chronic pain [4].
Photobiomodulation therapy (PBMT) also known as low-level laser/light therapy (LLLT), is a rapidly growing approach to stimulate healing [5, 6], increase tissue regeneration [6] and reduce pain [7] and inflammation [8]. PBMT has no known side-effects, and as yet has shown no long-term adaptation to the treatment [9, 10].
Typically, PBMT uses exposure of tissues to low-intensity light (power densities in the region of 1–100 mW/cm ) for a few minutes [9]. The wavelengths employed are mostly in the red or near-infrared (NIR) spectral regions, and although lasers were originally used exclusively, in recent years light emitting diodes (LED) have become popular [11]. During this exposure, light is absorbed by molecules in the cells called chromophores, such as cytochrome-c-oxidase, located inside mitochondria. Light absorption induces changes in cell signaling pathways and activation of transcription factors.
Application of NIR light for pain has clinical advantages such as deeper penetration, which allows noninvasive, transcutaneous
delivery [12]. Reports of adverse events in PBMT are extremely rare or non-existent, and this therapy has been approved by FDA (and other national health agencies around the world) for various indications [11].
Over the past decades, much PBM research has focused on demonstrating efficacy in animal models of potentially painful conditions. We previously reported [6] that PBM (810 nm laser, 300 mW/cm , 7.2 or 36 J/cm ) delivered to the head of mice could decrease the reaction to a potentially painful stimulus in the foot, evoked either by light pressure (von Frey filaments), cold plate, or inflammation (formalin injection), or in the tail (evoked by heat). Therefore, the goals of the present study were to establish a reliable treatment regimen (optimum bodily location of irradiation, and the optimum dose or energy density) and to investigate the mechanisms of action underlying the effect of light on pain threshold. The time of onset, duration, and repeatability of the procedure were investigated. This study also aimed to investigate the effects of PBM on neurological markers (mGluR1 and PAP) involved in pain signaling in the peripheral nervous system (the dorsal root ganglia, DRG).