Laser photobiostimulation-induced hypoalgesia in rats is not naloxone reversible

Laser photobiostimulation (LPBS) at the pulsing frequency of 4 Hz applied to the low resistance point located at the base of the tail of the rat, (Governing Vessel Meridian 1), produced a hypoalgesic effect, measured by tail-flick and hot-plate techniques. Pre-treatment with low dose naloxone (2 mg/kg) did not reverse the hypoalgesic effect of LPBS. High dose naloxone (20 mg/kg) reversed only partially, but significantly, the hypoalgesic effect of LPBS measured by hot-plate, but not that measured by the tail-flick technique. These data suggest that mechanisms other than endogenous opioids may be involved in LPBS-induced hypoalgesia.