Cell bodies of origin of serotonin-immunoreactive afferents to the inferior olivary complex of the rat.

Previous studies have used immunohistochemistry to localize serotonin to distinct olivary nuclei in several mammalian species. However, the location of the cell bodies of origin for the serotoninergic projection to the inferior olive in any of these species was unknown. In the present study, a paradigm which combines transport of horseradish peroxidase (HRP) and serotonin immunohistochemistry (PAP) was used to identify the cell bodies of origin of this afferent system to the inferior olivary complex of the rat. Cells which contain both retrogradely transported HRP and brown cytoplasmic staining indicating that they are serotoninergic cells that project to the inferior olivary complex are found exclusively in an area dorsal to the rostrolateral dorsal accessory olive within the nucleus reticularis paragigantocellularis. Neurons within this nucleus were also found to be a source of serotoninergic afferents to the cerebellum and spinal cord of the rat. This raises the possibility that individual serotonin-immunoreactive neurons within this nucleus may project to all 3 areas. Future studies will be designed to address this possibility. No double-labeled cells were observed within any of the raphe nuclei.