ETD

• rabies virus
• retrograde tracing
• serotonergic system

Serotonin (5-HT) is a monoamine neurotransmitter involved in the modulation of a wide range of physiological and behavioural processes. In the mammalian brain serotonin is produced by neurons clustered in the brainstem raphe nuclei (B1-B9). Despite their relatively reduced number, these neurons provide extensive innervation to the entire central nervous system.
Two different modalities of neurotransmission have been proposed to coexist within this system: wiring transmission (WT), through synaptic contacts, and volume transmission (VT), via extrasynaptic diffusion. While this dualism is commonly acknowledged, the relative distribution and functional relevance of WT and VT remain unclear.
This thesis investigates serotonergic innervation patterns using viral retrograde tracing tools. To map wiring-transmitting 5-HT neurons, I exploited a glycoprotein-deleted rabies virus (ΔG-mCherry-RV) in Tph2GFP mice, which selectively infects neurons at the presynaptic terminal.
To trace serotonergic projections independently of the presence of synapses, I exploited a pseudotyped RV in which the native glycoprotein is replaced by the EnvA protein (ΔG-EnvA-mCherry-RV). By using a Cre-dependent AAV helper virus to drive TVA expression in Pet1-Cre transgenic mice, serotonergic neurons were made susceptible to pseudotyped RV infection, thus overcoming the presynaptic specificity of the previous strategy.
Applying this approach across multiple brain regions, I demonstrated that each area exhibits distinct and specific proportions of serotonergic innervation in terms of wiring and volume components. These anatomical insights set the base for future functional studies to clarify the modalities of serotonergic transmission in the brain.