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 the 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 microelectrode 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.

Magnesium suppresses the responses of dorsal horn cell to noxious stimuli in the rat

Magnesium ion is known to selectively block the N-methyl-D-aspartate (NMDA)-induced responses and to have anticonvulsive action, neuroprotective effect and antinociceptive action in the behavioral test. In this study, we investigated the effect of Mg2+ on the responses of dorsal horn neurons to cutaneous thermal stimulation and graded electrical stimulation of afferent nerves as well as to excitatory amino acids and also elucidated whether the actions of Ca2+ and Mg2+ are additive or antagonistic. Mg2+ suppressed the thermal and C-fiber responses of wide dynamic range (WDR) cell without any effect on the A-fiber responses. When Mg2+ was directly applied onto the spinal cord, its inhibitory effect was dependent on the concentration of Mg2+ and duration of application. The NMDA- and kainate-induced responses of WDR cell were suppressed by Mg2+, the NMDA-induced responses being inhibited more strongly. Ca2+ also inhibited the NMDA-induced responses current-dependently. Both inhibitory actions of Mg2+ and Ca2+ were additive, while Mg2+ suppressed the EGTA-induced augmentation of WDR cell responses to NMDA and C-fiber stimulation. Magnesium had dual effects on the spontaneous activities of WDR cell. These experimental findings suggest that Mg2+ is implicated in the modulation of pain in the rat spinal cord by inhibiting the responses of WDR cell to noxious stimuli more strongly than innocuous stimuli.