Differential projections of cat medullary raphe neurons demonstrated by retrograde labelling following spinal cord lesions

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Abstract
Neurons of the medullary raphe nuclei in cats were retrogradely labelled following injection of horseradish peroxidase (HRP) into the L6 spinal cord segment. Brainstems were cut in sagittal section to facilitate examination of the rostral‐caudal extend of raphe neurons projecting to the spinal cord. Large numbers of HRP‐labelled neurons were found in nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus (as well as a few neurons in nucleus raphe pontis). Dorsal or ventral hemisections at the T12‐L1 level restricted HRP retrograde transport to those pathways within the intact portion of spinal cord, allowing a determination of the part of the cord through which raphe neurons project to the lumbar enlargement. Neurons of nucleus raphe magus were found to project primarily in dorsolateral fasciculus. A significant number of neurons of nucleus reticularis gigantocellularis also project in dorsolateral fasciculus. Nucleus raphe obscurus neurons were found to project primarily in ventral funiculus, while nucleus raphe pallidus neurons project in the ventrolateral fasciculi and ventral funiculus. The serotonergic (5HT) fibers described by Dahlström and Fuxe ('65) to terminate in the dorsal horn, intermediolateral cell column, and ventral horn are likely to coincide with the raphe‐spinal projections documented in this work.