The topographical distribution of serotoninergic terminals in the neostriatum of the rat and the caudate nucleus of the cat

Abstract

The topographical distribution of serotoninergic terminals in the neostriatum of the rat and the caudate nucleus of the cat was established owing to the combined use of microdissection techniques and biochemical microassays. The density of 5-HT terminals in various areas of both structures was quantified first by measuring 5-HT levels in microdiscs of frozen tissue. Since the high affinity uptake process for 5-HT appeared undamaged in isotonic homogenates of previously frozen (--5 degrees C) tissues, it was possible to confirm the findings obtained with the measurement of 5-HT levels by also determining 5-HT uptake activity in these microdiscs. However, in the rat neostriatum, but not in the cat caudate nucleus, [3H]5-HT even at a very low extracellular concentration (4.4 -x 10(-8) M) was taken up not only by serotoninergic terminals but also to a significant extent by dopaminergic terminals. In presence of benztropine, this second component was suppressed and [3H]5-HT uptake activity could then be considered as a specific marker of serotoninergic terminals also in the neostriatum of the rat. In both species, 5-HT terminals were mainly localized in the ventrocaudal area of the structure. In this area, 5-HT levels were among the highest values found in the brain (17 ng/mg protein). The density of 5-HT terminals decreased progressively from the acudal to the rostral planes of the neostriatum in rats or the caudate nucleus in cats. The poorest area, i.e. the dorsorostral zone, contained about 4 times less 5-HT than the ventrocaudal zone of the structure. Electrolytic lesion of the dorsalis (B7) and centralis superior (B8) raphe nuclei during early life resulted in a large decrease of 5-HT levels (--90%) in various parts of the neostriatum of adult rats. The present findings might be of interest to further analyze the role of serotoninergic neurons in extrapyramidal functions.