Possible role of the nucleus raphe dorsalis in analgesia by peripheral stimulation: theoretical considerations

Direct stimulation of the Nucleus Raphe Dorsalis (NRD) appears to be very effective for obtaining analgesia. However, the possible role of the NRD in analgesia by peripheral stimulation, e.g. electro-acupuncture or Transcutaneous Electrical Nerve Stimulation (TENS), has received less attention. Most studies indicate that in particular the nucleus reticularis gigantocellularis, the nucleus raphe magnus, the nucleus reticularis magnocellularis, the locus coeruleus, and the periaqueductal grey may participate in the supraspinal mechanism for the antinociceptive effect of peripheral stimulation. This paper is meant to present some theoretical considerations about the role of the NRD might play in analgesia by peripheral stimulation. Little direct evidence suggests that the NRD/serotonergic system might contribute to this type of analgesia. The analgesic role of specifically the serotonergic neurons of the NRD in peripheral stimulation could be supported by indirect evidence. From studies concerning direct stimulation of the NRD, two other mechanisms by which peripheral stimulation might exert antinociceptive effects through the NRD have been hypothesized. Stimulation-parameters affecting the effectiveness of peripheral NRD stimulation will be discussed.