Microglia depletion in early life programs persistent changes in social, mood-related, and locomotor behavior in male and female rats

• We used liposomal clodronate to deplete microglia in neonatal rats and study the lifelong effects on motivated behavior.
• Central liposomal clodronate injection at P1 and P4 depleted forebrain microglia for approximately two weeks.
• Early-life microglia depletion led to increased locomotion and decreased anxiety and despair behaviors throughout life.
• Females that had microglia depleted neonatally had a blunted corticosterone response to acute stress in adulthood.

Microglia, the innate immune cells of the central nervous system, regulate brain development by promoting cell genesis, pruning synapses, and removing dying, newly-born or progenitor cells. However, the role of microglia in the early life programming of behavior under normal conditions is not well characterized. We used central infusion of liposomal clodronate to selectively deplete microglia from the neonatal rat brain and subsequently assessed the impact of microglial depletion on programming of juvenile and adult motivated behaviors. Liposomal clodronate treatment on postnatal days one and four led to greater than 70% loss of forebrain microglia by postnatal day 6 that lasted for approximately ten days. Neonatal microglia depletion led to reduced juvenile and adult anxiety behavior on the elevated plus maze and open field test, and increased locomotor activity. On a test of juvenile social play, microglial depletion led to decreased chase behaviors relative to control animals. There was no change in active social behavior in adults on a reciprocal social interaction test, but there was decreased passive interaction time and an increased number of social avoidance behaviors in clodronate treated rats relative to controls. There was an overall decrease in behavioral despair on the forced swim test in adult rats treated neonatally with clodronate. Females, but not males, treated neonatally with clodronate showed a blunted corticosterone response after acute stress in adulthood. These results show that microglia are important for the early life programming of juvenile and adult motivated behavior.