Spinal Serotonergic and Opioid Receptors Are Involved in Electroacupuncture-Induced Antinociception at Different Frequencies on ZuSanLi (ST 36) Acupoint

The present study was conducted to evaluate the effect of electroacupuncture-(EAc-) induced antinociception (EAA) at different currents and frequencies in rat spinal cord. We found that naloxone (0.05 μg i.t.) blocked EAA at different frequencies. Naltrindole (0.05 μg i.t.) blocked EAA on the 7th day after EAc of 100 Hz. 5,7-Dihydroxytryptamine (100 μg i.t.) significantly inhibited EAA at different frequencies on the 7th day after EAc. Pindobind (0.5 μg i.t.), a 5-HT(1A) antagonist, notably attenuated EAA at different frequencies. Ketanserin (0.5 μg i.t.), inhibited EEA at a lower frequency (<10 Hz) than at a higher frequency (100 Hz). LY-278584 (0.5 μg i.t.) significantly inhibited EAA at a higher frequency (100 Hz) on the 7th day after EAc. The direction of effect of 8-OH-DPAT, on EAA was dependent on dosage. It had an inhibitory effect at a low dose (0.5 μg i.t.) and a high frequency (100 Hz) but enhanced EAA at a higher dose at lower frequencies (<10 Hz). DOI (10 μg, i.t.), did not affect EAA. These data indicate that the mechanism of EAA involves opioid receptors, and the serotonergic system, particularly, μ-, δ-opioid and 5-HT(1A), 5-HT(3) receptors and it is also dependent on the EAc frequency.