Electro-acupuncture analgesia in monkeys: a behavioral and neurophysiological assessment

Monkeys were initially trained to press a lever to escape noxious electrical stimulation applied to one leg. They were then presented with electrical stimuli consisting of 40 successive intensity increments beginning at zero intensity and with random interstimulus intervals (ISIs). A lever press, initiated by the monkey during presentation of a stimulus, immediately terminated stimulation of the leg, triggering another sequence of the same increasing stimuli with a new series of ISIs. After establishment of a stable threshold measure of lever pressing, 45 min of electro-acupuncture was applied to traditional points on both legs. The results show that electro-acupuncture dramatically increased (300-400 %) the lever pressing threshold for approximately 20 min after electro-acupuncture stimulation was terminated. In acute monkey preparations, spike activity of single neurons in N. Parafascicularis of the thalamus, evoked by noxious electrical stimulation to the sciatic nerve, was recorded. The evoked activity of only nociceptive specific neurons was either eliminated or markedly reduced for at least one hour following 45 min of electro-acupuncture applied to traditional points on both legs. The observations made by these two experimental approaches suggest that electro-acupuncture exerts an analgesic effect on artificially induced nociception.