Involvement of Peripheral Opioid Mechanisms in Electroacupuncture Analgesia

The involvement of the peripheral opioid system in modulating inflammatory pain has been well documented. This study aimed to investigate the possibility of electroacupuncture (EA)-mediated peripheral opioid release. Rats were injected with complete Freund's adjuvant in one of the hind paws to induce localized inflammatory pain. The pain behavioral changes were measured by paw withdrawal latency (PWL) to a noxious thermal stimulus. At day 5 of inflammation, rats received a second injection of saline or opioid antagonists into the inflamed paw, followed by EA at 30 Hz, 2 mA, and 0.1 ms for 30 minutes. The EA was conducted at acupuncture point GB30. A control was used in which needles were inserted at GB30 but no electrical stimulation was applied. Rats receiving EA showed a significantly longer PWL as compared with the control from 30 minutes to three hours after EA treatment. Intraplantar but not intraperitoneal injection of naloxone methiodide, a peripherally acting opioid receptor antagonist, eliminated the analgesic effect at 30 minutes after EA treatment. Intraplantar injection of an antibody against β-endorphin and a corticotropin-releasing factor antagonist also produced a reduction in PWL in rats receiving EA. These data strongly suggest that peripheral opioids are released by EA at the inflammatory site.