Serotonergic projections to the spinal cord from the midbrain in the rat: an immunocytochemical and retrograde transport study.

Neurons localized in the midbrain nucleus raphe dorsalis and caudal midbrain reticular formation have direct projections to the cervical spinal cord in the rat. By means of double labeling technique, using the retrograde cell marker horseradish peroxidase and immunocytochemistry, 90% of these spinally projecting neurons were positively stained for serotonin immunoreactivity. These results indicate that serotonergic innervation of the spinal cord is provided not only by the medullary cell groups B1-B3, but also by the midbrain cell groups B7 and B9.