Vesicle-containing dendrites in the nucleus raphe dorsalis of the cat. A serial section electron microscopic analysis

The nucleus raphe dorsalis of the cat was examined by serial section electron microscopy and the presence of vesicle-containing dendrites is reported. Such dendrites were divided into two classes according to their synaptic contact. Dendrites containing round and/or pleomorphic vesicles associated with a clear synaptic specialization which was generally intermediate between a Gray's type I or II were classified as presynaptic dendrites. These presynaptic dendrites were presynaptic to conventional dendrites and dendritic spines. In addition, some profiles containing a sparse population of vesicles which may be dendritic in nature were observed involved in serial synaptic arrangements. A second class of dendrites were characterized by the presence of vesicles which were never found associated with any synaptic membrane specialization. Commonly, the vesicles were densely packed and associated with unusual densities. Serial section analysis of these densities showed that they were not presynaptic dense projections. We suggest that the existence of vesicle-containing dendrites in the nucleus raphe dorsalis of the cat constitute the morphological support for the dendritic release of neurotransmitters.