Immune cells in the brain control 'rough-and-tumble' social behaviour in male adolescent rats, according to a study published in Nature Communications. These findings help to better understand sex-specific behaviour and how the brain develops just before adulthood.

Recently, scientists have discovered that a type of immune cell called microglia helps sculpt developing brain circuits. Soon after birth, microglia engulf or 'eat' synapses (junctions at which neurons communicate), pruning away superfluous synapses that form in infant brains. The properties of microglia can differ between males and females, but scientists are still exploring how this changes behaviour and later development.

Ashley Kopec, Staci Bilbo and colleagues found that near adolescence, male rats show a brief spike in play behaviour such as pouncing, pinning each other down, or rolling onto their backs. The authors found that microglia end this period of increased play in males by engulfing synapses that detect dopamine, a chemical 'reward' signal in the brain. These synapses are tagged for elimination by a molecule called C3. In females, social play does not peak during adolescence, but microglia still seem to affect play behaviour, just not by engulfing dopamine-detecting synapses.

More research is needed to explain how microglia change female play and it is unknown if microglia also regulate social behaviour in humans. However, the authors suggest these findings may provide insights into the teenage brain and sex-specific development, and could lead to future insights into brain diseases that arise in early adulthood or differ between sexes.