Early-life Inflammation Promotes Depressive-like Behaviors In Adolescent Mice

An early-life inflammatory event during brain development,such as trauma or viral infections experienced during pregnancy and childhood,has been shown to strongly increase the risk for mood disorders(e.g.,anxiety and depression)in adolescence or adulthood.However,the underlying mechanisms are largely unknown.The clinical studies have shown that the anterior cingulate cortex(ACC)of depressed patients is accompanied by decreased synaptic density and elevated levels of inflammatory cytokines,and the level of inflammation is positively correlated with the severity of depression.Microglia are the resident Immune cells of the brain.Activated microglia are closely associated with the development of depression as the commanders of the inflammatory state.However,it is unknown whether the pattern of microglial activation and altered neuronal activity modulation contribute to depressive symptoms in adolescence,which have experienced early-life inflammation during adolescent development.In the present study,combining three-dimensional(3D)reconstruction,electrophysiology,in vivo fiber photometry recording and two-photon imaging,chemogenetics,and pharmacological methods,we investigated how microglia in the ACC of mice with early-life inflammation regulate neuronal activity and contribute to the development of depressive-like behaviors in adolescence.A mouse model of early-life inflammation was established through intraperitoneal injection of LPS on postnatal day(P)14.Next,we investigated the pattern of ACC microglial response to stress and the mechanism of depressive-like behaviors generation during the development of adolescent mice(P14-P45).We found that multiple activated markers of microglia were increased in the ACC at 6 hours after LPS injection,including Iba1,CD68 and MHCII,which returned to a normal level after 24 hours.In addition,the LPS-treated mice showed significant depressive-like behaviors compared with saline-treated mice,and was accompanied by reactivation of ACC microglia,suggesting that the microglia of ACC in LPS-treated mice are susceptible to stress.To validate this result,we conducted a series of unpredictable stressful events(e.g.,weaning,cage separation,noise exposure,and fighting)during adolescent development,and found that all of them can lead to a reactivation of ACC microglia in LPS-treated mice.Meanwhile,the depressive-like behaviors of LPS-treated mice were significantly alleviated upon inhibition(administered with the microglial inhibitor,Minocycline)or elimination(fed chow containing PLX3397 for 3 weeks)of microglia in the ACC.Activity-dependent neuroplasticity is one of the basis that regulate animal behaviors.Our further studies revealed that when stress occurred,the activity of ACC glutamatergic(ACCGlu)neurons were dramatically increased to resist the stress and exerted a protective effect.Indeed,we found that chemogenetic activation of ACCGlu neurons in LPS-treated mice could alleviate depressive-like behaviors on P45.However,sustained stressful events led to frequent activation of ACC microglia during adolescent development of the mice with early-life inflammatory experiences.Such microglial activation mediated excessive engulfment of ACCGlu neuronal dendritic spines through CX3CR1 signaling,resulting in a long-term maladaptive state manifested as reduced ACCGlu neuronal activity.Consequently,the insufficient activation of ACCGlu neurons during the presence of stress in LPS-treated mice,causes poor coping outcomes to stressful challenges and thus promotes the development of depressive-like behaviors in adolescent mice.In summary,this study found that early-life inflammation mediates depressive-like behaviors in adolescent mice by promoting the microglial engulfment of neuronal dendritic spines.Thus,our data provides a experimentally tractable framework to understand the molecular,cellular basis for how microglia regulate neuroplasticity and postnatal inflammatory events manifest much later as adolescent depression.