Radioautographic study of in vivo incorporation of 3H-monoamines in the cat caudate nucleus: identification of serotoninergic fibers.

The localization of radioactivity was examined in the caudate nucleus of the cat, treated with an MAO inhibitor, following local superfusion of the ventricular surface of the structure with low concentration of 3H-catecholamines or [3H]5-HT. The caudate nucleus was superfused continuously from 30 to 240 min using a cup technique. Light microscope or high resolution radioautographs revealed: (1) a rather diffuse incorporation of 3H-catecholamines under the subependymal region which could be hardly attributed to a specific population of nerve terminals. A loose binding of 3H-catecholamines into dopaminergic terminals could be involved in this effect. (2) An intense and selective uptake of [3H]5-HT in fine scarce varicose nerve fibers localized in the ventricle as in the subependymal layer and in the neuropil of the nucleus. The labeled fibers contained numerous round or elongated large granular vesicles of 80-120 nm and exhibited only very few synaptic contacts suggesting a possible extrasynaptic liberation of 5-HT.