The serotonergic system modulates complex brain functions in anatomically widely distributed target regions. Elucidation of the topographical organization of serotonergic projections and the potentially regionally specific regulation of serotonergic neurotransmission has remained a major challenge for neuroscientists. The anatomical organization of serotonergic systems illustrates the complexity of the problem. Individual neurons are highly collateralized and project to diverse regions (Lowry, 2002). Moreover, they contact other serotonergic neurons and maintain negative-feedback loops (Barnes & Sharp, 1999; Sharp et al., 2007; Jensen et al., 2008); such interactions between separate groups of serotonergic neurons are likely to have major impacts on synaptic mechanisms and behavior.
Serotonergic neurons are clustered in the brainstem (including the reticular formation and raphe nuclei), and were originally subdivided into the B1–B9 cell groups (Dahlstrom & Fuxe, 1964). The most prominent part of this complex is the dorsal raphe nucleus. The dorsal raphe nucleus corresponds to the original subdivisions B6–B7, and can be divided into five subregions: the interfascicular, ventromedial, ventrolateral, dorsal and caudal subregions (Baker et al., 1990). Each of these subregions has been shown to project to specific parts of the cerebral cortex and subcortical regions, and consequently may differentially regulate serotonergic neurotransmission (Steinbusch et al., 1981; O’Hearn & Molliver, 1984). Moreover, these brainstem nuclei receive descending projections from multiple forebrain regions (Hajos et al., 1998; Peyron et al., 1998; Celada et al., 2001). However, past studies using conventional anterograde and retrograde tracing methods have only partially elucidated the projection patterns of the different groups of serotonergic neurons. Injection sites were not always comparable between studies, and uptake of tracer by neighboring and neurochemically different cells limited the interpretation of the evidence from these experiments.
In their article published in this issue of EJN, Bang et al. employed an innovative strategy to genetically dissect the projections of different subgroups of serotonergic neurons in the raphe complex. They applied intersectional genetic tools with triple transgenic mice to identify the functional heterogeneity and projection pattern of serotonergic neurons based on their developmental origin and associated gene expression history. The authors generated three lines of mice in which enhanced green fluorescent protein expression was selectively driven by serotonergic neurons originating from distinct rhombomeric segments, and demonstrated that these cells form distinct subgroups in several raphe nuclei and have differential patterns of projection to forebrain areas. Detailed comparison of the three lines of mice revealed partial overlap between the projections from serotonergic neurons with distinct developmental origins. Most importantly, the results also demonstrated that these three serotonergic cell populations maintain differential connections with other serotonergic nuclei. Therefore, negative feedback on the activity of specific subsets of serotonergic cells does not seem to be a unitary feature of all these subgroups.
The results of Bang et al. begin to resolve some of the discrepancies reported by studies analyzing effects of various experimental manipulations on extracellular serotonin levels across projection target regions. Individual serotonergic neurons project to restricted target regions and are likely to release serotonin in a regional-specific manner. Clearly, however, the main significance of the results of Bang et al. concerns the unambiguous demonstration that specific serotonergic subgroups are strongly interconnected and modulate each other. Thus, these results advance our understanding of the organization and function of subgroups of neurons in the raphe complex. This information is crucial for elucidating the neurobiological basis of the numerous pathological conditions associated with dysfunction of serotonin neurotransmission, notably depressive and anxiety symptoms (Cowen, 2008). Further investigations with the techniques presented in this study will provide the neuroanatomical substrate to specifically target the different parts of the 5-hydroxytryptamine system, not only with existing therapies also with new therapeutic approaches, such as deep brain stimulation.