Tolerance to electroacupuncture analgesia was reversed by microinjection of 5-hydroxytryptophan into nuclei accumbens in the rabbit.

Electroacupuncture (EA) of the hind limb points of the rabbit for 10 min elicited a significant increase of the latency of aversive responses (ARL) induced by radiant heat stimulation of the snout. This effect of analgesia was found to be gradually diminished when EA was applied repeatedly for 6 hr, implying the development of tolerance. A reversal of EA tolerance was achieved by microinjection of 5-hydroxytryptophan (5-HTP) (the direct precursor of 5-hydroxytryptamine (5-HT)), either intracerebroventricularly (200 micrograms) or into bilateral nuclei accumbens (10 micrograms). Injection of the same amount of 5-HTP into brain structures adjacent to nucleus accumbens, e.g., globus pallidus, corpus callosum, internal capsule, etc., was ineffective. It is concluded that a functional insufficiency of 5-HT in the brain, especially in nucleus accumbens, may constitute one of the mechanisms for the development of EA tolerance.