Opposing roles of microglial and macrophagic C3ar1 signaling in stress-induced synaptic and behavioral changes

The social deficits following chronic stress conditions are linked to synaptic dysfunction in the brain. Complement system plays a critical role in synapse regulation. Although complement has been implicated in chronic stress-induced behavior deficits the cellular substrates and mechanisms underlying complement-mediated behavior changes under chronic stress conditions are not known. In the present study, we investigated the role of complement component 3a receptor (C3ar1) in microglia and monocytes/macrophages (Mo/MΦ) in chronic unpredictable stress (CUS)-induced synapse loss and behavior deficits in mice. We found that deletion of microglial C3ar1 attenuated stress-induced social behavior deficits and changes in neuroinflammatory as well as synaptic markers in the prefrontal cortex (PFC). RNA sequencing data revealed that microglial C3ar1 deletion attenuates CUS-mediated changes in the expression of immediate-early genes such as Fos and Nuclear Receptor Subfamily 4 Group A Member 1 (Nr4a1) in the PFC. In contrast, lack of C3ar1 in Mo/MΦ induced social behavior deficits. Together, these findings indicate opposite functions of C3ar1 signaling in microglia and Mo/MΦ under chronic stress conditions.