Effect of electroacupuncture tolerance by different frequencies on the opioid inhibition to cardiovascular activity in the spinal cord of rats.

Continuous 2-15 Hz or 2 Hz electroacupuncture (EA) of 6 h applied to rats resulted in electroacupuncture tolerance (ET). The effect of 2-15 Hz ET on the recovery of mean arterial blood pressure (MAP) and heart rate (HR) of rats after hemorrhagic shock, and the effect of intrathecal (i.t.) injection of [D-ala2, D-leu5]enkephalin (DADLE) on MAP and HR of 2 Hz ET rats were observed. The results were as follows: 1) there was no dramatic difference between the 2-15 Hz ET group and the control group in the recovery of MAP and HR from hemorrhagic shock; 2) DADLE (25 micrograms) caused almost the same depressor effect in the 2 Hz ET group as that in the control group. These data suggest that the spinal lateral horn cells (supposed to be involved in regulating blood pressure) are insensitive to EA and cannot be made tolerant to continuous EA, which is different from the dorsal horn cell (supposed to be involved in analgesia).