Dynamic changes of serotonin transporter expression in the prefrontal cortex evoked by aggressive social interactions.

Aggression is a complex behavior influenced by developmental experiences, internal state, and social context, yet its neurobiological underpinnings remain insufficiently understood. The serotonergic system, particularly the serotonin transporter (SERT), plays a crucial role in aggression regulation. Here, we investigated region-specific, dynamic changes in SERT expression following aggressive interactions and in mice subjected to early-life social adversity. We found that aggressive encounters (resident-intruder test) triggered a significant, rapid increase in SERT immunoreactivity within 90 min, accompanied by neuronal activation in aggression-related brain regions, including the medial prefrontal cortex (mPFC), lateral septum (LS), medial amygdala (MeA), ventromedial hypothalamus (VMHvl), lateral habenula (LH), and dorsal raphe (DR), but not in the paraventricular thalamus (PVT). Notably, this SERT upregulation occurred across the aggression circuitry but was accompanied by a significant increase in 5-HT levels only in the mPFC, a key region in top-down regulation of social and aggressive behavior. This SERT upregulation was not observed following exposure to a non-social challenge, suggesting that it may be more specifically associated with social contexts. Using super-resolution microscopy, we identified an increased density of SERT localization points within serotonergic mPFC axons after an aggressive encounter. Social isolation during adolescence, a model of early social neglect, impaired this rapid SERT response, particularly in the ventral and medial orbitofrontal regions, and altered the relationship between SERT levels and aggression-related behaviors. These findings demonstrate that SERT expression undergoes rapid, experience-dependent plasticity in response to social aggression, and that early-life adversity disrupts this adaptive mechanism, providing new insights into the serotonergic regulation of aggression and its potential relevance for stress-related social dysfunctions.