Analgesia induced by electroacupuncture of different frequencies is mediated by different types of opioid receptors: another cross-tolerance study.

The cross-tolerance technique was used to analyze the receptor mechanisms of analgesia induced by electroacupuncture (EA) of 2 Hz, 100 Hz, or 2-15 Hz. (1) Rats were given EA stimulation of 2 Hz, 100 Hz and 2-15 Hz for 30 min with 30 min intervals successively. The percentage increase in tail-flick latency (TFL) was taken to indicate the intensity of EA analgesia. Rats made tolerant to repeated intrathecal injection of the mu-opioid agonist ohmefentanyl (OMF, 15 pmol, Q2h x 5) or the delta-opioid agonist DPDPE (10 nmol, Q2h x 5) showed a cross tolerance to both 2 Hz- and 2-15 Hz-, but not to 100 Hz-EA analgesia; and rats made tolerant to kappa-opioid agonist dynorphin-(1-13) (5 nmol, Q2h x 5) showed a cross-tolerance to 100 Hz- and 2-15 Hz-, but not to 2 Hz-EA analgesia; (2) Rats made tolerant to 2-15 Hz EA showed cross-tolerance to either 2 Hz- or 100 Hz-EA analgesia; (3) Rats made tolerant to either 2 Hz- or 100 Hz-EA were still reactive to 2-15 Hz-EA. The results indicate that 2 Hz-EA analgesia is mediated by mu- and delta-receptors, 100 Hz-EA analgesia by kappa-receptor, and 2-15 Hz-EA analgesia by combined action of mu-, delta- and kappa-receptors in the spinal cord of the rats.