PROJECT SUMMARY Serotonin is an evolutionarily conserved neurotransmitter that modulates the activity of excitatory and inhibitory neurons throughout the entire mammalian brain and is thus essential for diverse aspects of physiology and behavior. Drugs that impact the serotonin system have been used to treat numerous brain disorders including depression, anxiety, and post-traumatic stress disorder. In the mammalian brain, serotonin neurons are clustered in the raphe nuclei of the brainstem, but project axons across the entire brain. We lack a fundamental understand of how serotonin neurons are organized to achieve their diverse functions. Our recent studies suggested that dorsal raphe (DR) serotonin neurons likely comprise parallel subsystems with distinct projection patterns, input biases, physiological response properties, and behavioral functions. For example, DR serotonin neurons that project to orbitofrontal cortex and central amygdala have distinct collateralization patterns, receive quantitatively biased monosynaptic inputs from a diverse set of brain regions, respond oppositely to punishment, and have distinct functions in promoting active coping and anxiety behaviors, respectively. Single-cell transcriptomic profiling revealed that DR serotonin neurons comprise 7 transcriptomic types, distinct from the 4 transcriptomic types in the nearby median raphe (MR) serotonin neurons. Our unpublished data on collateralization mapping further indicated diverse, complex, yet stereotyped collateralization patterns of DR and MR serotonin neurons. Here we propose to use a combination of approaches, including viral-genetic access of serotonin neurons that project to specific brain regions, whole-brain mapping of axon collateralization patterns, in situ transcriptomic and projectomic typing, chemogenetic and optogenetic manipulations, fiber photometry and Neuropixels-based physiological recordings, and statistical modeling. Specifically, we will complete the collateralization mapping and divide serotonin neurons into specific subsystems. We will determine the behavioral functions of a subset of serotonin systems. We will also characterize the dynamics of a subset of serotonin subsystems and the effect of their action on target neuron dynamics. By combining viral genetic dissection of serotonin subsystems with anatomical, physiological, and behavioral analyses, our proposed studies have the potential to take a major step forward in our understanding of the organization and function of the serotonin system in the mammalian brain.

Project Narrative In the mammalian brain, serotonin neurons are clustered in brainstem but project axons across the entire brain and modulate diverse physiological and behavioral functions. We propose to use viral-genetic approaches to access specific serotonin populations based on their projectomic and transcriptomic profiles to record and manipulate their activity. Our approaches will help dissect the complex serotonin systems into specific subsystems with diverse physiological functions and behaviors.