Morphological evidence for selective modulation by serotonin of a subpopulation of dorsal horn cells which possess the neurokinin-1 receptor

Serotonin selectively depresses transmission of nociceptive information thr ough the spinal dorsal horn but the mechanisms of this depression are poorl y understood. In this study we report that serotonin-containing axons form basket-like clusters which are intimately woven around cell bodies and prox imal dendrites of a subpopulation (approximate to 50%) of laminae III/IV ne urons which possess the neurokinin-1 receptor. Statistical analysis confirm s that cells belonging to this subpopulation have significantly higher numb ers of serotoninergic contacts on proximal dendrites when compared with the population of neurokinin-l cells that are not associated with clusters (me an +/- SD = 13 +/- 5.8 and 5 +/- 2.9, respectively). Neurokinin-1 cells in laminae III/IV project to regions of the brain which are involved in nocice ptive processing and are likely to be activated predominantly by nociceptiv e input. The concentration of serotoninergic axons around proximal regions of some of these cells indicates that serotonin may have a powerful influen ce on transmission through this pathway. This type of arrangement could be a morphological correlate for at least part of the selective antinociceptiv e actions of serotonin.