The implication of central serotonin in electro-acupuncture tolerance in the rat.

Electro-acupuncture (EA) applied to both legs for 30 min raised the tail flick latency of the rat significantly. This effect of EA analgesia became attenuated and finally disappeared after repeated EA stimulations, which was termed EA tolerance. EA tolerance and its cross tolerance to morphine analgesia were found to be partially reversed by intracerebro-ventricular injection of 5-hydroxytryptophan (5-HTP), the precursor of 5-hydroxytryptamine (5-HT). However, there was no depletion in cerebral 5-HT, nor was there decrease of the number of 5-HT receptors in the brain of the EA tolerant animals. Results obtained from different lines of analysis point to the conclusion that development of tolerance to profoundly released 5-HT during prolonged EA stimulation may constitute one of the mechanisms for EA tolerance.