ELECTRICAL-STIMULATION AT TRADITIONAL ACUPUNCTURE SITES IN PERIPHERY PRODUCES BRAIN OPIOID-RECEPTOR-MEDIATED ANTINOCICEPTION IN RATS

Previous studies in rats measuring latency to tail flick with radiant heat have shown that the antinociceptive effect induced by electrical stimulation of different frequencies at traditional acupuncture sites is mediated via different opioid receptors in the spinal cord. The pre sent study was designed to observe (1) whether electrical stimulation at such sites could produce antinociceptive effects in the cold water tail-flick (CWT) test; (2) whether the antinociceptive effects could b e blocked by s.c. injection of the opioid receptor antagonist naloxone and (3) whether i.c.v. injection of selective antagonists for mu (cyc lic D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, CTAP), delta (naltrindole ) or kappa (nor-binaltorphimine) opioid receptors would block the anti nociceptive effect produced by electrical stimulation. Sprague-Dawley rats were stimulated at frequencies of 2, 30 or 100 Hz with acupunctur e needles inserted into acupoints Zusanli and Sanyinjiao in the hind l eg for 30 min. Antinociception was assayed in the CWT. The results sho wed that (1) a significant, frequency-related increase in threshold in the CWT was observed in all electrical stimulation groups as compared with the placebo group and the antinociceptive effect lasted about 30 min poststimulation; (2) naloxone (s.c.) antagonized the antinocicept ive effect induced by 2 Hz, 30 Hz or 100 Hz electrical stimulation and (3) either CTAP or naltrindole (i.c.v.) almost completely blocked the antinociceptive effect induced by 2 Hz or 30 Hz electrical stimulatio n, but was less effective in blocking antinociception induced by 100 H z electrical stimulation; nor-binaltorphimine (i.c.v.) greatly reduced antinociception induced by 30 Hz or 100 Hz electrical stimulation, bu t not by 2 Hz electrical stimulation. These results indicate that the antinociception induced by 2 Hz electrical stimulation is mediated by both mu and delta opioid receptors; the antinociception induced by 100 Hz electrical stimulation is mediated primarily by the kappa receptor ; and the antinociception induced by 30 Hz electrical stimulation is m ediated by all three opioid receptor types. Thus, the antinociceptive effect induced by peripheral electrical stimulation, as measured by th e CWT, involves opioid receptors in the rat brain.