Serotonin innervation in adult rat neostriatum. I. Quantified regional distribution

The distributional features of the serotonin (5-HT) innervation in adult rat neostriatum were examined and quantified using two complementary chemoanatomical methods: 5-HT-immunohistochemistry on serial histological sections and radioautography after [3H]5-HT uptake in whole cerebral hemisphere slices. As visualized and measured after peroxidase-antiperoxidase immunostaining, the neostriatal 5-HT fiber network pervading the entire neostriatum was 2-3 times denser in its ventral than dorsal parts, and showed a slight rostrocaudal increase in density. Its axonal length ranged from 1.06 to 4.18 m per mm3 of striatal tissue. Radioautographic counts of the [3H]5-HT-labeled axon varicosities within comparable sectors of the neostriatum showed good correlation with this distribution pattern. As extrapolated after appropriate corrections for incomplete detection at the chosen exposure time and from the thickness of sections examined, the number of neostriatal 5-HT varicosities (innervation density) ranged from 1.5 to 4.8 millions and averaged 2.6 millions per mm3 of tissue. These quantitative results provided new insights into the topographical organization of the dorsal raphe-neostriatal 5-HT projection system. They already allow for meaningful correlations with currently available microchemical data on intrastriatal 5-HT levels and should also be of considerable significance when as precise information becomes available on the number and localization of different 5-HT receptors and uptake carriers within rat neostriatum.