OBJECTIVETo observe the analgesic effects of single-and combined-laser irradiation with low-intensity applied at "Zusanli" (ST 36) in rats, and their relation to degranulation of mast cells.

METHODSSixty-six SD rats were randomly divided into 6 groups: normal control group (Group NC), model control group (Group MC), sham irradiation group (Group SI), 10.6 microm laser irradiation group (Group 10.6 microm LI), 650 nm laser irradiation group (Group 650 nm LI) and combined (10.6 microm + 650 nm) laser irradiation group (Group CLI). Complete Freund's Adjuvant (0.05 mL) was injected into the left ankle joints of all the rats except those in Group NC to cause acute adjuvant-induced arthritis. In treatment, laser irradiation was applied at "Zusanli" (ST 36) for 30 minutes in all the rats except those in Group NC and Group MC. The paw withdrawal latency (PWL) to radian heat was used to compare analgesic effects among the groups. By means of toluidine blue, dyed slices of local tissues of "Zusanli" (ST 36) were used to observe changes of mast cell degranulation before and after laser irradiation.

RESULTSThe pain thresholds to irradiation of the rats in Group 650 nm LI and Group CLI were significantly higher than those in Group MC and Group SI (P < 0.01), and the mast cell degranulation rate in Group 650 nm LI and Group CLI were also significantly higher than that in Group MC and Group SI (P < 0.001). The pain threshold and mast cell degranulation rate in Group 10. 6 microm LI were not significantly different from those in Group MC and Group SI. There was a linear correlation between mast cell degranulation rate and PWL with 0. 737 in coefficient (P < 0.001).

CONCLUSIONSingle 650 nm laser and combined 650 nm + 10.6 microm laser with low intensity irradiated at "Zusanli" (ST 36) in acute adjuvant rats can provide remarkable analgesic effects, and there was a positive correlation between mast cell degranulation rate and analgesic effects, which plays an important part in laser irradiation-induced analgesia.