Origins of serotonergic projections to the lumbar spinal cord in the monkey using a combined retrograde transport and immunocytochemical technique.

Abstract

A newly developed technique employing the retrograde transport of horseradish peroxidase combined with immunocytochemistry is used to identify the cells of origin of descending spinal pathways and their putative neurotransmitters. With this technique the brainstem origins of descending serotonergic (5HT) pathways to the lumbar spinal cord have been determined in the monkey. Numerous 5HT stained neurons are found in the nucleus raphe obscurus and raphe magnus and in the adjacent reticular formation projecting to the lumbar spinal cord. The nucleus raphe pallidus contains relatively fewer descending 5HT neurons. In addition to the spinally projecting neurons containing 5HT, large multipolar shaped neurons within the raphe nuclei were found to project to the spinal cord, but these do not stain for 5HT immunoreactivity. These findings indicate that the raphe nuclear complex provides both serotonergic and non-serotonergic inputs to the spinal cord. The advantages and uses of the present double labeling method for localizing other neurotransmitter substances in identified neuronal pathways are discussed.