Mapping study of serotoninergic input to diencephalic‐projecting dorsal column neurons in the rat

Several lines of evidence indicate that the processing of somatosensory information in the dorsal column nuclei (DCN) is subject to descending controls. Anatomical experiments have demonstrated projections to the DCN from the sensorimotor cerebral cortex and the reticular formation. Physiological studies have shown that the activity of DCN neurons can be altered following stimulation of the cerebral cortex, reticular formation, periaqueductal gray, or raphe nuclei. Recent biochemical and electrophysiological evidence suggests a serotoninergic modulation of DCN neurons. The present study identifies serotonin‐containing contacts on cells in the DCN that project to the thalamus in the rat. Retrograde labeling of brainstem neurons by horseradish peroxidase demonstrated projections to the DCN from the nucleus reticularis paragigantocellularis lateralis and from several raphe nuclei, including nuclei raphe obscurus (RO), pallidus (RP), and magnus (RM). Double labeling with horseradish peroxidase and antibody for serotonin indicated that the RO, RP and RM are likely to be the sources of the serotoninergic projections to the DCN. Thus, the role of the serotoninergic output from the raphe nuclei includes modulation of activity in the DCN.