Projection-defined median raphe Pet subpopulations are diversely implicated in seizure.

The raphe nuclei, the primary resource of forebrain 5-HT, play an important but heterogeneous role in regulating subcortical excitabilities. Fundamental circuit organizations of different median raphe (MR) subsystems are far from completely understood. In the present study, using cell-specific viral tracing, Ca fiber photometry and epilepsy model, we map out the forebrain efferent and afferent of different MR Pet subpopulations and their divergent roles in epilepsy. We found that Pet neurons send both collateral and parallel innervations to different downstream regions through different subpopulations. Notably, CA3-projecting Pet subpopulations are largely distinct from habenula (Hb)-projecting Pet subpopulations in anatomical distribution and topological organization, while majority of the CA3-projecting Pet subpopulations are overlapped with the medial septum (MS)-projecting Pet subpopulations. Further, using Ca fiber photometry, we monitor activities of Pet neurons in hippocampal-kindling seizure, a classical epilepsy model with pathological mechanisms caused by excitation-inhibition imbalance. We found that soma activities of Pet neurons are heterogeneous during different periods of generalized seizures. These divergent activities are contributed by different projection-defined Pet subpopulations, manifesting as increased activities in CA3 but decreased activity in Hb resulting from their upstream differences. Together, our findings provide a novel framework of MR subsystems showing that projection-defined MR Pet subpopulations are topologically heterogenous with divergent circuit connections and are diversely implicated in seizures. This may help in the understanding of heterogeneous nature of MR 5-HTergic subsystems and the paradox roles of 5-HTergic systems in epilepsy.