[Projection of the mesencephalic raphe nuclei to the magnocellular nuclei of the basal forebrain of the rat: a horseradish peroxidase study].

In 15 rats the projection of the mesencephalic raphe nuclei to the magnocellular nuclei of the basal forebrain (b.f.) were investigated by using the HRP-method. In 5 other rats the distribution of serotoninergic fibers in the b.f. was analyzed immunocytochemically. The most important results are: The projection courses ipsilaterally only. The number of labelled cells comprehends only 5-30% of the number of cells estimated in the raphe nuclei that were studied. In the Ncl. r. linearis oralis only a few cells were labelled after injection of HRP into the subnuclei of the diagonal band. More cells were labelled after injection of HRP into the more caudal lying nuclei of the b.f. In the Ncl. r. centralis superior, more labelled cells could be observed after HRP-injection into the rostral nuclei of the b.f. (Ncl. tr. diagonalis with pars septalis, pars verticalis and pars horizontalis; Ncl. praeopticus magnocellularis) than after injection into the caudal nuclei. The labelled cells were of all the types described in the literature. In the Ncl. r. dorsalis, most of the labelled cells were observed in the medial part of this nucleus. In the lateral part there were only a few labelled cells. After injection of HRP into the caudal nuclei of the b.f. (Subst. innominata, medial and ventral boundary of the Gl. pallidus) more cells were labelled than after injection into the more rostral nuclei of the b.f. Of the cell types of the Ncl. r. dorsalis, the medium-sized and large multipolar types and the medium-sized fusiform cells were most frequent labelled. The Ncl. r. centralis superior tended to project more to the rostral nuclei, and the Ncl. r. dorsalis more to the caudal nuclei of the magnocellular b.f. The serotinergic fibers appeared more or less loosely distributed in the nuclei of the b.f. We could not differentiate between subtypes of the serotoninergic axons.