Dendrite bundles in nuclei raphe dorsalis and centralis superior of the rabbit: a possible substrate for local control of serotonergic neurons.

A Golgi-Cox, cresyl violet, and histofluorescence study has revealed the presence of dendrite bundles in nuclei raphe dorsalis (NRD) and centralis superior (NCS) in the rabbit brain stem. In NRD, bundles were found in the midline, traversing the medial longitudinal fasciculus (MLF) and in a circumferential location around the MLF. In NCS, bundles were found oriented vertically in the midline. Serotonergic dendrites predominated in these bundles, but non-serotonergic dendrites from cells of the dorsal tegmental nucleus, adjacent reticular formation, and NCS also were present. Long descending shafts from tanycytes on the floor of the rostral fourth ventricle were also found in the dendrite bundles of both NRD and NCS. We suggest that the dendrite bundles constitute a local neuronal system for regulating the activity of these raphe neurons.