Anatomic evidence suggestive of dendrodendritic synapses in the opossum basilar pons

In Golgi-prepared material, opossum basilar pontine intrinsic neurons measured less than 22 microns and gave origin to only two or three primary dendrites which gradually tapered in diameter, branched infrequently and exhibited few spines or protrusions. Characteristically, such neurons gave rise to several axon-like processes which might take origin from proximal or distal dendrites as well as the soma. Morphologically similar neurons have been observed in several other regions throughout the CNS where they have been shown to be a source of presynaptic dendritic elements. That opossum basilar pontine intrinsic neurons might also give rise to presynaptic dendrites was supported by the following electron microscopic observations. Profiles containing a small cluster of pleomorphic vesicles, occasional ribosomes and numerous microtubules formed synaptic active sites in which the membrane densities were intermediate between Gray's type I and type II. Such presumed presynaptic dendritic profiles were observed to participate in serial synaptic arrangements in which they were always postsynaptic to a round vesicle bouton and presynaptic to another dendritic element. The above features are compatible with electron microscopic descriptions of presynaptic dendrites in other CNS regions and thus suggest that basilar pontine intrinsic neurons may represent a source of presynaptic dendritic elements.