Pre-Irradiation of Blood by Gallium Aluminum Arsenide (830 nm) Low-Level Laser Enhances Peripheral Endogenous Opioid Analgesia in Rats

BACKGROUND: Low-level laser therapy (LLLT) has been reported to relieve pain, free of side effects. However, the mechanisms underlying LLLT are not well understood. Recent studies have also demonstrated that opioid-containing immune cells migrate to inflamed sites and release beta-endorphins to inhibit pain as a mode of peripheral endogenous opioid analgesia. We investigated whether pre-irradiation of blood by LLLT enhances peripheral endogenous opioid analgesia. METHODS: The effect of LLLT pretreatment of blood on peripheral endogenous opioid analgesia was evaluated in a rat model of inflammation. Additionally, the effect of LLLT on opioid production was also investigated in vitro in rat blood cells. The expression of the beta-endorphin precursors, proopiomelanocortin and corticotrophin releasing factor, were investigated by reverse transcription polymerase chain reaction. RESULTS: LLLT pretreatment produced an analgesic effect in inflamed peripheral tissue, which was transiently antagonized by naloxone. Correspondingly, beta-endorphin precursor mRNA expression increased with LLLT, both in vivo and in vitro. CONCLUSION: These findings suggest that that LLLT pretreatment of blood induces analgesia in rats by enhancing peripheral endogenous opioid production, in addition to previously reported mechanisms.