Activation of spinal anti-analgesic system following electroacupuncture stimulation in rats.

We evaluated the interaction between electroacupuncture (EA)-induced antinociception and an endogenous anti-analgesic system. EA was applied to the ST-36 acupoint for 45 min in male Sprague-Dawley rats, and pain thresholds were assessed by the hind-paw pressure test. EA produced a marked increase in pain thresholds and its antinociceptive action was completely reversed by naloxone (5 mg/kg). The analgesic effects of subcutaneous morphine (7 mg/kg) following EA stimulation were significantly attenuated. The attenuation of morphine analgesia was inversely proportional to the time intervals between EA termination and morphine injection, and the effect was not observed 120 min after EA stimulation. The analgesic effects of i.t. morphine (10 microg), but not i.c.v. morphine (25 microg), following EA were also attenuated. On the other hand, systemic morphine (7 mg/kg)-induced hyperthermia was not affected by EA. Moreover, i.c.v. morphine, but not i.t. morphine, produced hyperthermia. The i.c.v. morphine-induced hyperthermia was not affected by EA, similar to i.c.v. morphine analgesia. These results suggest that the attenuation of morphine analgesia following EA, that is, the activation of an endogenous anti-analgesic system, is closely related to the activation of an analgesic system by EA and that the spinal cord plays a critical role in the activation of the endogenous anti-analgesic systems.