Abstract
It was postulated by Kaada in 1974 [I] that acupuncture analgesia was produced by a humoral factor activating the descending midline ‘serotonin system’ from the raphe nuclei known to exert inhibitory effects on spinal pain transmission. The present crucial experiment, the elimination of acupuncture analgesia by selective lesions of these nuclei, has supported this assumption.
As nociceptive stimulation, electrical shocks were applied to the feet. Analgesia (reduced jump responses) was obtained by 2 sec−1 trigeminal stimulation. The effect came on gradually during the 60 min stimulation period with return to the original value in about 1 hr, i.e. about the same time course as for morphine or acupuncture analgesia. Both effects are blocked by naloxone, indicating the involvement of the endogenous opiate system.
Selective electrolytic lesions of the nucleus raphe magnus (projecting to the spinal cord) as well as the nucleus raphe dorsalis or centralis superior (both projecting to the forebrain) eliminated the trigeminal-induced analgesia. Raphe magnus lesions reversed the response to foot shocks during trigeminal stimulation, suggesting the presence of a facilitatory pain system unveiled by removal of the raphe inhibition. Lesions of the nucleus raphe magnus or centralis superior both lowered the initial response level to foot shocks, possibly due to interruption of ascending fibers of the paleo-spinothalamic tract passing through this area.
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