Microglia depletion in a mouse model of prenatal and postnatal immune activation

Risk for neurodevelopmental disorders can be related to early immune stimulations and altered brain microglial functions. Here we investigated the behavioural and electrophysiological effects of microglia depletion in a mouse model of developmental immune activation. C57BL/6J pregnant mice were exposed on gestational day 12.5 to polyinosinic:polycytidylic acid (Poly I:C), subsequently, on postnatal day 9, offspring was further treated with lipopolysaccharide (LPS). At weaning, offspring was exposed throughout adolescence (4 weeks) to either a diet containing Colony Stimulating Factor-1 receptor inhibitor (PLX5622, PLX) to reduce microglia, or standard diet. Hence, we assessed i) explorative and anxiety-like responses, social responsiveness and cognitive abilities between 7^th and 8^th postnatal week; ii) synaptic transmission and neuroinflammatory and microglial molecular markers in the medial prefrontal cortex (mPFC) and hippocampus (HP) at the end of treatment (8^th postnatal week). EIA condition reduced locomotor activity and impaired discrimination between a familiar and a novel social stimulus (social novelty response) only in male mice. Also, PLX treatment selectively affected the same social novelty response in males (both saline and EIA) and in EIA females, intriguingly sparing saline females. Unexpectedly, EIA condition per se did not affect spontaneous excitatory and inhibitory synaptic transmission in both mPFC and HP, whereas EIA combined with PLX reduced inhibitory transmission in males (both mPFC and HP) and neuron excitability in both male and female mPFC. Interestingly, PLX had per se sex- and region- specific effects increasing inhibitory transmission in female mPFC and decreasing excitatory transmission in male HP. Molecular data, beside a robust downregulation of microglia markers in PLX diet groups, also showed that EIA condition increased interleukin-6 (il-6) expression in EIA males in both mPFC and HP, and elevated il-1β levels in both sexes in mPFC and in male HP. Overall, these findings indicate that males have an increased vulnerability to the long-term behavioural and inflammatory effects of the EIA condition, and are more likely to exhibit behavioural and electrophysiological changes in response to microglia depletion.