Ultrastructure of descending serotoninergic axonal endings in layers I and II of the dorsal horn.

Serotoninergic axonal endings in layers I and II of the medullary and spinal dorsal horn of the cat were ultrastructurally characterized following uptake of [3H]serotonin. The two most common types were dome-shaped endings which could be distinguished by the size and shape of their agranular synaptic vesicles. The two types of endings were found throughout layers I and II, but were most numerous in layer I. The labeled endings formed both symmetrical and asymmetrical synapses on dendritic shafts and spines, and occasionally a symmetrical synapse on a neuronal soma. Using the technique of anterograde transport of [3H]amino acid, two morphologically identical types of endings were demonstrated as originating from neurons in the caudal raphe nuclei and adjacent reticular formation.