From B1 to B9: A guide through hindbrain serotonin neurons with additional views from multidimensional characterization

Abstract

Brain serotonin (5-HT) is produced by hindbrain neurons localized in the raphe nuclei.Although these neurons share a common molecular identity and some structural andphysiological characteristics, the 5-HT raphe neurons differ according to their rostro-caudaland mediolateral position in the brainstem, which corresponds to different developmentalorigins. This defines to a large extent, differences in molecular profiles, anatomicalconnectivity and physiology. Recent efforts for a systematic and multidimensionalcharacterization of the 5-HT raphe neurons at the single cell level allowed to individualizefunctionally different 5-HT raphe cells, that can in part cross the border of the classicsubdivisions. In this chapter, we start with an overview the embryonic development of raphecell groups, and summarize the current view of how 5-HT raphe neurons are subdivided intodifferent anatomical, neurochemical, and functional entities. We use the B1-B9 nomenclatureas an entry point to systematically overview cytoarchitecture, connectivity and co-neurotransmission properties in each of these raphe 5-HT cell groups.