Topographic organization and chemoarchitecture of the dorsal raphe nucleus and the median raphe nucleus

The role of serotonergic systems in regulation of behavioral arousal and sleep-wake cycles is complex and may depend on both the receptor subtype and brain region involved. Increasing evidence points toward the existence of multiple topographically organized subpopulations of serotonergic neurons that receive unique afferent connections, give rise to unique patterns of projections to forebrain systems, and have unique functional properties. A better understanding of the properties of these subpopulations of serotonergic neurons may aid in the understanding of the role of serotonergic systems in regulation of behavioral arousal, sleep-wake cycles and other physiological and behavioral responses attributed to serotonin. In this chapter, we outline evidence for multiple serotonergic systems within the midbrain and pontine raphe complex that can be defined based on cytoarchitectonic and hodological properties. In addition, we describe how these topographically organized groups of serotonergic neurons correspond to the six major ascending serotonergic tracts innervating the forebrain.