Light and electron microscopic immunocytochemical analysis of the serotonin innervation of the rat visual cortex

The serotonin afferents of the rat visual cortex were examined immunocytochemically at the light and electron microscopic levels. Immunoreactive fibres were typically thin, tortuous and varicose. Occasionally, some thicker fibres were found. The orientation of labelled axons varied according to laminar position, with fibres running parallel to the pial surface present mainly in layers I and VI, and radially oriented fibres prominent in layers II and III. Branches arising from horizontal or radially oriented fibres were seen to form irregularly shaped loops particularly in layers IV and V. The density of innervation and the prevailing axonal orientation in each cortical layer were similar in both coronal and parasagittal planes. The ultrastructural features of serotonin-labelled axon terminals were examined in single and serial ultrathin sections. While in single sections the majority did not exhibit synaptic specializations, extensive serial section analysis showed that virtually all of these terminals were engaged in junctional complexes. Postsynaptic elements were spines and dendritic shafts, including pyramidal cell apical dendrites, with both symmetrical and asymmetrical membrane specializations. In axospinous synapses, the labelled terminals were usually adjacent to unstained axon terminals contacting the same postsynaptic element.