Antinociception produced by 2 and 5 KHz peripheral stimulation in the rat

Recently a "medium frequency" electric stimulator (LIKON) was marketed, which delivers 2000-5000 Hz square waves and was claimed to induce a strong clinical analgesic effect. The aim of this study was to clarify whether electric stimulation over 1000 Hz would induce any antinociceptive effect in the rat. Tail flick latency (TFL) was taken as the nociceptive index. Electrical stimulation was applied via stainless steel pins inserted into the acupoint ST 36 located near the knee joint and SP 6 located near ankle joint on both hind legs. Significant increase of TFL (ANOVA, p < .01) was obtained after 2 or 5 kHz stimulation for 10 min, and lasted for the whole stimulation period of 30 min. No significant difference was observed whether the amplitude remained constant (5 V) or was modulated at 5 or 100 cycles/s. Opioid antagonist naloxone (20 but not 10 mg/kg, sc) produced a 50% blockade of the antinociception induced by 2 kHz, but not 5 kHz stimulation. The results suggest that 2 kHz peripheral stimulation was capable of inducing an opioid mediated antinociception in the rat, whereas a less marked antinociception induced by 5 kHz stimulation was nonopioid in nature.