Generation and characterization of chemogenetic mouse models for the study of the brainwide effects induced by serotonergic neurotransmission manipulation

Serotonin (5-HT) neurotransmission affects behaviors and neuro-physiological functions via the orchestrated recruitment of distributed neural systems. Human imaging studies have employed pharmacological manipulations to probe the brainwide substrates targeted by serotonin. However, systemic drug manipulations often lack neural and receptorial specificity, and result in combined central and peripheral contributions that cannot be easily disentangled. As a result, it remains unclear whether and how serotonergic activity specifically influences regional or global functional activity. Here we combine DREADD-based chemogenetics and mouse fMRI, an approach we term “chemo-fMRI”, to causally probe the brainwide substrates endogenously modulated by phasic 5-HT neurons stimulation. To this aim, I generated of two conditional knock-in mouse models that, crossed with Pet1-Cre-transgenic mice, allowed us to remotely stimulate serotonergic transmission during fMRI scans. I show that chemogenetic stimulations of 5-HT system results in a composite pattern of activation encompassing parieto-cortical, hippocampal, and midbrain structures, as well as ventro-striatal components of the mesolimbic reward systems. Many of the activated regions also exhibit increased c-Fos immunostaining upon stimulation in freely-behaving mice, supporting a neural origin of the observed activation. Collectively, these findings identify a set of regional substrates that act as primary functional targets of endogenous serotonergic stimulation, and establish causation between phasic activation of 5-HT neurons and regional fMRI signals. They further highlight a functional cross-talk between 5-HT and mesolimbic dopaminergic, and provide a novel framework for understanding 5-HT-dependent functions and interpreting data obtained from human fMRI studies of serotonin modulating agents.