ORGANIZATION, DEVELOPMENT, AND PLASTICITY OF THE SEROTONERGIC DORSAL RAPHE NUCLEUS

The dorsal raphe nucleus (DRN) is the main source of serotonin (5-HT) in the central nervous system. Through their widespread and heterogeneous projections, 5-HT DRN neurons modulate a wide range of physiological functions and brain states. Yet, the organizational logic, developmental dynamics, and experience-dependent plasticity of 5-HT DRN projections remain poorly understood. Here, we combined single-neuron arborization tracing, whole-brain registration, longitudinal imaging, and environmental manipulations to map the complete 5-HT DRN projectome in larval zebrafish and study its developmental and experience-dependent restructuring. By reconstructing 81 individual serotonergic neurons across developmental stages, we revealed that the 5-HT DRN neurons exhibit diverse projection patterns, ranging from broadly distributed to spatially confined trajectories. We also identified two largely non-overlapping projection-defined subpopulations, targeting either telencephalon or spinal cord. In addition, 5-HT DRN projections display a partial topographic organization, with certain brain regions being preferentially innervated by specific DRN subregions. Longitudinal analysis revealed that the overall organization of the DRN projectome, as well as the projection profiles of individual neurons, remain generally stable during early development, although neurite refinements occur. Furthermore, exposure to early life stress during the first week of development, but not thereafter, altered the 5-HT DRN projections in a region-specific manner, suggesting a temporally restricted window of heightened plasticity. Overall, our study provides a comprehensive view of the spatial organization, developmental dynamics, and stress-dependent plasticity of the 5-HT DRN projectome in the vertebrate brain.