Serotonin modulates nucleus accumbens circuits to suppress aggression in mice

Serotonin (5-hydroxytryptamine; 5HT) has long been considered anti-aggressive, but the mechanisms by which 5HT regulates downstream circuits to control aggression remain unclear. Combining fiber photometry, optogenetics, and miniaturized microscope recordings in double-transgenic male mice, we find that 5HT levels ramp up in the nucleus accumbens during aggression, inhibiting a subset of D1 medium spiny neurons to suppress attacks. Our results reveal a novel 5HT-mediated neuromodulatory mechanism for limiting aggressive behavior. Serotonin is thought to reduce aggression, but the mechanisms have been unclear. Here, authors show that when a mouse attacks an intruder, serotonin levels rise in the nucleus accumbens, terminating the attack by inhibiting specific aggression signalling cells.