Serotoninergic fibers in dorsal roots of the spinal cord.

In the lumbar segment of the spinal cord, serotonin-containing nerve fibers are shown to connect the descending serotoninergic pathway of the intermediolateral column with the ipsilateral dorsal root. This may suggest that efferent serotoninergic fibers, originating from brainstem raphe nuclei, leave the spinal cord via the dorsal roots. However, when tryptophan pretreatment is employed in addition to a monoamine oxidase inhibitor, a limited number of yellow-fluorescing neurons can at times also be visualized in the dorsal ganglia. These could be the cell bodies of origin of at least some of the above-mentioned serotonin-containing nerve fibers, which would then have, at least in part, an afferent, sensory function. At present, therefore, the origin and function of this newly discovered serotoninergic pathway remains to be clarified.