High-resolution radioautographic study of the serotonin innervation of the rat corpus striatum after intraventricular administration of [ 3H]5-hydroxytryptamine

Abstract

In the rat striatum, identification of the serotonin-containing nerve endings at the ultrastructural level has been done by radioautography after ventriculodsternal perfusion of [ 3H]serotonin. Only the ventricular border of the middle part of the nucleus caudatus-putamen (neostriatum) was studied and, in addition, a small dorsal part of the globus pallidus. For every 100 μm 2of thin sections of the neostriatum, 0.5 labelled structures were found, which were either axons (14%), preterminal enlargements (21.5%) or nerve terminals (64.5%). Three kinds of labelled nerve endings could be recognized according to the size and shape of their synaptic vesicles, although all contained occasional large granular vesicles. The first type (36%) was characterized by small and often granular, ovoid vesicles mixed with tubular structures of the same diameter (20 nm). The second type (28%) contained crowded spherical vesicles of 45 nm in diameter which were mostly clear. The third type (36%) contained a mixture of both types of synaptic vesicles. The labelled nerve terminals in the neostriatum showed a few synaptic contacts (mean of 14%) which were, however, more numerous in the second type of nerve endings (25–30%) and mainly of the asymmetric type. In the globus pallidus the serotonin-labelled nerve terminals resembled those in the neostriatum but one additional type was observed, which was characterized by only a few typical synaptic vesicles and by numerous large granular vesicles. The observations made in this study indicate that the serotonin-containing varicosities of the striatum are very similar to those described in some other nuclei. They exhibit characteristic large granular vesicles and the synaptic vesicles often have an eccentric tiny granule after administration of exogenous serotonin and/or inhibition of monoamine oxidase. The peculiar type of nerve endings with tubular vesicles described by others in the cerebellum and in the locus coeruleus was also present in the neostriatum. The morphological heterogeneity of serotonin-containing nerve endings within a single nucleus or from one nucleus to another, is discussed in relation to the amine content, the stage of formation of the synaptic vesicles and their origin in different raphe nuclei.