A rabies virus based approach to map serotonergic neurons innervating different brain structures

Serotonergic neurons are part of one of the most widely distributed neural systems in the mammalian brain (Lauder and Bloom, 1974). Serotonergic neurons form the raphe nuclei in the brain stem, and are organized in distinct nuclei (B1-9) that project to the whole central nervous system. Consistently with such a broad innervation, serotonin is involved in a wide range of physiological processes including the control of appetite, sleep, memory, mood, stress and sexual behavior (Veenstra-Vanderweele et al, 2000). Several studies using anatomical tracing methods and anterograde viral tracers have led to the hypotheses of a topographic organization of the serotonergic system, with different projections from the caudal, median/central and dorsal raphe neurons to specific target districts in the rostral brain (Muzerelle et al. 2014). Experimental evidence suggests that clusters of serotonergic neurons within the raphe nuclei may have distinct functional properties, but the complex organization of serotonergic neuron projections remains poorly understood. The aim of the present study is to map at a finer scale the organization of serotonin neurons projecting to different target areas, thus contributing to understanding the functional role of specific serotonergic neuronal subpopulations. To this end, we used a Tph2::GFP knock-in mouse model, in which serotonergic neurons are clearly labeled by the expression of GFP (Migliarini et al, 2013), and the retrograde recombinant rabies viral tracer to map the serotonergic neurons innervating different brain structures. Moreover, we developed a conditional GFP expressing mouse model, in which the reporter is maintained under the transcriptional control of the Tph2 gene and activated upon an flp mediated somatic recombination, to map the organization of serotonergic neuron subgroups sharing common targets in the brain