The Effect of Low-Level Laser Irradiation on Activities of Dorsal HOrn Neurons in Rats with Experimental Muscle Pain

Objective: This study was performed to investigate effects of low-level laser irradiation on the dorsal horn cell activities in the rats with experimental muscle pain. Method: Experimental muscle pain was induced by repeated injections of acidic saline into the gastrocnemius muscle in thirty Sprague-Dawley rats. Activities of ipsilateral dorsal horn cells were recorded with a glass microelec trode at L2-L4 spinal cord level before and after low-level laser irradiation on the injected leg. Cells were categorized as wide dynamic range (WDR) and high threshold (HT) cells by the response to the cotton, vonFrey filament, and forceps stimulation at the receptive area. Results: The spontaneous and mechanically evoked activities of WDR and HT cells were significantly increased in the muscle pain models. Low-level laser irradiation reduced mechanically evoked activities of WDR and HT cells. This effect was maximal at 20 minutes after irradiation and then returned to pre-treatment level in 40-50 minutes. Conclusion: The low-level laser irradiation was effective for the management of muscle pain by reducing activities of dorsal horn cells. Low-level laser might be clinically used for the treatment of local and referred pain of muscle origin.