• Serotonin
• Retrograde tracing
• Rabies virus
• Connectome

Serotonin-synthetizing neurons, which are confined in the raphe nuclei of the rhombencephalon, provide a profuse innervation network throughout the central nervous system and are involved in the modulation of a plethora of brain functions. In the last few years, the development of novel genetic tools and high-throughput techniques has added unforeseen degrees of heterogeneity among serotonergic neurons in terms of neurodevelopmental origins, gene programs and electrophysiological properties. Recently, a map of the complex topographical organization of the serotonergic fibers has been drawn using intersectional fate mapping strategy, as well as retrograde or anterograde tracing. However, no comprehensive retrograde study has been performed yet, and anterograde approaches still set a series of limitation. Here, combining the recombinant G-deleted rabies virus system with Tph2GFP knock-in mice, in which serotonergic neurons were clearly labeled by the expression of GFP, I present a map of the topographic organization of 5-HT ascending projection to 19 rostral brain regions, known to be regulated by 5-HT. This experimental approach revealed that each brain district hereby investigated is innervated by a relatively small and region-specific number of serotonergic neurons. Thanks to Cre-independent and -dependent monosynaptic tracing, coupling pseudotyped recombinant rabies virus with a helper adeno-associated virus, it was possible to deduce that such narrow subpopulation of 5-HT neurons is characterized by a wiring transmission mode. Moreover, the balance between wiring and volume transmission appears to be region-specific, and may underlie specific regulation mechanism on neuronal functions. Altogether, my results can shed new lights on the communication properties of the serotonergic system, highlighting a further level of serotonergic control of brain function, possibly helping to better understand its role in health and disease.