Radioautographic study of 5-hydroxytryptamine-containing nerve terminals in central ganglia of Planorbis corneus: comparison with other species and characteristics of the serotoninergic nerve terminal.

Central ganglia from Planorbis corneus were incubated with [3H]5-hydroxytryptamine and [3H]-leucine. After fixation, sections were examined by light and electron microscopic autoradiography. With [3H]5-hydroxytryptamine there was a high level of uptake into a small number of axons and terminal processes in the neuropil. The terminal processes contained granular vesicles (diameter 50--120 nm), some of which possessed eccentrically placed cores, and agranular vesicles (mean diameter 60 nm). No membrane synaptic specializations were observed. With [3H]leucine there was a general distribution of radioactivity throughout the ganglia. The vesicular inclusions and non-specialized nature of the labelled terminal processes were very similar to presumed 5-hydroxytryptamine-containing terminals in other invertebrates and in the mammalian brain. It appears that several anatomical features of serotoninergic nerve terminals are common in all animals studied, although there is no specific characteristic which allows positive identification.