Goodwin_2026_eNeuro__

Aggression may be behaviorally distinguished by reactive or appetitive properties. Here, we use a model of operant aggression administration, in which outbred male CD-1 mice lever press (contingent) or do not lever press (non-contingent) to attack an intruder mouse, to examine behavioral differences in aggression reinforcement. Contingent reinforcement identifies the behavioral and neural basis of appetitive, or rewarding, aggression self-administration, while non-contingent reinforcement isolates reactive, or involuntary, components. Females are not used in this study due to their low propensity to attack. We applied supervised machine-guided behavioral classification and Shapley additive scores (SHAP) to describe differences and similarities in attack behavior features. We find that behavioral sequences of an attack bout are similar whether aggression reinforcement is contingent or non-contingent, though underlying neural mechanisms differ. Fos immunolabeling following operant reinforcement reveals distinct network activity patterns between contingent and non-contingent groups, supporting distinct neural mechanisms in appetitive or reactive aggression. We further identify high Fos activity in nucleus accumbens in both contingent and non-contingent groups. Nucleus accumbens (NAc) activity and changes in NAc neuroligin-2 (NLGN2) expression are associated with aggressive behavior. We find that the number of attack bouts is negatively correlated with NAc NLGN2 immunolabeling, regardless of contingent or non-contingent intruder presentation. As a result, molecularly dissociable features do not necessarily reflect operant behavioral repertoires.Significance Statement: Aggressive behavior presents for different reasons, and two important clinical presentations include appetitive (sought after) and reactive (fight or flight) aggression. Understanding how these phenotypes differ in their behavioral and neural signatures is essential for identifying mechanisms at the intersection of neuropsychiatric disease and maladaptive aggression. Using an operant model in outbred male CD-1 mice, we combined explainable machine learning and forebrain Fos mapping to compare contingent (appetitive) and non-contingent (reactive) aggression. We find that, although behavioral repertoires are notably similar across contingency contexts, neural activity and nucleus accumbens neuroligin-2 expression distinguish aggression severity and phenotype. These findings reveal that molecular features can diverge from behavior, providing new insight into the neurobiology of contingent and non-contingent aggression.