Analgesic effects of cranial laser treatment in two rat nociception models.

The present experiments sought to establish dose dependency and time course for effects of cranial laser irradiation in two rodent models of pain. These were the hot plate and tail flick tests, which are both widely used to quantify analgesic drug effects. The laser used was an Omega Biotherapy 3ML (wavelength 820 nM, average power output 100 mW, pulse frequency 5 kHz) and irradiation was applied to rats' shaved heads above the midbrain. In the first experiment, four groups of 10 rats received doses of 0, 6, 12, 18, and 24 J/cm2 in random orders prior to hot plate testing either immediately, 30 min, 1 h or 24 h postlaser. The second study employed three groups of 10 rats receiving 0, 12, and 18 J/cm2 in random orders prior to tail flick testing at the three shorter times above. Latency to lick hind paws on the hot plate was highly significantly prolonged by laser treatment across all doses and time periods, F(4, 126) = 4.51, p < 0.01. There was good dose dependency for immediate observations, but at 24 h 18 J/cm2 was the most effective dose. Laser treatment also delayed tail flick responses at both doses and all time periods, F(2, 54) = 10.60, p < 0.001, but 12 and 18 J/cm2 doses were similar in efficacy.