Retrosplenial Cortex is Involved in Analgesia Induced by 2- but not 100-Hz Electroacupuncture in the Rat Tail-Flick Test

This study examined whether or not the antinociceptive effect of 2- or 100-Hz electroacupuncture (EA) depends on the integrity of the retrosplenial cortex (RSC). Rats were taken for determination of tail-flick latency before and after injection of saline or 2% lidocaine (0.25μl) into the retrosplenial cortex (RSC) bilaterally. Five minutes later, they were submitted to a 20-minute period of 2Hz, 100Hz, or sham EA at the Zusanli and Sanyinjiao acupoints bilaterally, and tail-flick latency was measured within 30 seconds after the end of stimulation and at 5-minute intervals for up to 30 minutes. EA at a frequency of either 2 or 100Hz induced a strong and long-lasting inhibition of the tail-flick reflex in rats treated with saline (0.25μl) injected into the RSC. The analgesia produced by 2-Hz EA lasted for a shorter time in lidocaine-treated rats. By contrast, RSC impairment did not change the analgesic effect of 100Hz EA. The integrity of the RSC is implicated in the duration of analgesia induced by low-frequency EA but is not essential for the analgesic effects evoked by high-frequency EA.