Mapping the projections of serotonergic dorsal raphe neurons in zebrafish

The dorsal raphe nucleus (DRN) contains the largest population of serotonergic (5-HT) neurons in the central nervous system. The DRN modulates a wide range of physiological functions and brain states, mostly due to its widespread and heterogeneous projections. Evidence suggests that the DRN comprises different sub-systems with distinct neurotransmitter phenotype, connectivity and function. These sub-systems are difficult to characterize in the complex mammalian DRN. To overcome this, we address these questions in zebrafish larvae, leveraging their simpler and conserved DRN.Here, we establish a whole-brain map of 5-HT DRN neurons’ projections. To achieve this, we trace the neurites of individual neurons using 1 week-old transgenic zebrafish, in which the expression of green fluorescent protein is targeted to the 5-HT DRN neuron’s membrane.Our results show that collectively, 5-HT DRN neurons innervate most of the brain. Yet, 5-HT neurons display diverse projection patterns, with some neurons widely innervating many brain regions and others exhibiting more confined projections. Multiple brain regions receive extensive innervation from 5-HT DRN neurons, including the hypothalamus, spinal cord and thalamus . Furthermore, 5-HT DRN neurons project predominantly to ipsilateral brain regions. Additionally, we identified a subpopulation of 5-HT DRN neurons co-expressing GABAergic markers. We are currently studying how the innervation pattern of 5-HT DRN neurons develops from 1 to 2 weeks of age, and how exposure to chronic stress during that period may affect this process.Overall, our study provides a comprehensive view of the organization, development and plasticity of the 5-HT system in a vertebrate brain.