Analgesic effect of Ga‐Al‐As diode laser irradiation on hyperalgesia in carrageenin‐induced inflammation

This study concerned the effect of Ga‐Al‐As diode laser irradiation (780 nm, continuous wave, 31.8 J/s/cm2, spot size od 0.2 mm, 3 minutes/dose) on hyperalgesia induced in the hind paw of rats by injecting carrageenin. The pressure‐pain thresholds of hind paws were measured by the Randall‐Selitto test for evaluation of hyperalgesia. Two doses of laser irradiation, given to the inflamed region immediately before and after the injection of carrageenin, partially (∼ 50%) inhibited the occurrence of hyperalgesia accompanied with a progression of inflammation. This analgesic effect was equal to that of indomethacin (4 mg/kg, i.o.). In another group, the hyperalgesia was removed almost completely for at least 24 hours by one dose of laser irradiation, which was given 3 hours after the carrageenin injection, whereas the edema was not inhibited. This analgesic effect, however, was partially (∼ 50%) antagonized with a dose of 10 mg/kg (i.p.) of naloxone and totally inhibited with 30 mg/kg. These results suggest that low‐power laser irradiation on inflamed regions of carrageenin‐treated rats has a marked analgesic effect and that certain mechanisms that are not related to endogenous opioids are involved in a part of the mechanisms of the analgesic effects. © 1993 Wiley‐Liss, Inc.