Ultrastructure of the terminals of an identified 5-hydroxytryptamine-containing neurone marked by intracellular injection of radioactive 5-hydroxytryptamine.

The axons and terminal processes of an identified 5-hydroxytryptamine-containing neurone were located for study with the electron microscope. Tritiated 5-hydroxytryptamine was injected into the neurone's cell body,, and after allowing time for the radioisotope to pass along the processes of the neurone, tissue was examined by electron microscope autoradiography. Terminal processes were observed at approximately 1 cm from the injected cell body, in areas of the nervous system where previous electrophysiological work had shown the presence of postsynaptic neurones. Dense-cored vesicles of mean diameter 100 nm were observed in the axons and terminal processes of the neurone. The terminal processes also contained aggregates of clear vesicles of mean diameter 65 nm. Structures resembling lysosomes, which were frequently labelled, were present in the axons of the neurone. No membrane specializations associated with typical synapses were observed.