Aversion But Not Aggression: Emotional Traits of KM Rats in Sociability Tests

The inbred Krushinsky-Molodkina (KM) rat strain, characterized by latent genetic epilepsy, reduced social motivation, and a high propensity for freezing, is a promising model for translational research of social deficits in autism spectrum disorder (ASD). In clinical practice, social deficits often lead to social withdrawal and can be accompanied by aggression. It was unknown if KM rats exhibit such aggression during inescapable social encounters. We assessed intraspecific aggression in KM and control rats using the resident-intruder test, and autonomic nervous system responses were evaluated via heart rate variability analysis in a small separate cohort using a modified social-challenge paradigm that prevented direct physical contact. In the resident-intruder test, KM rats displayed significantly less aggression toward unfamiliar intruders than controls, exhibiting fewer attacks, fights, and competitive wins. Electrocardiographic analysis during the social preference test further revealed that the social challenge triggered parasympathetic, rather than sympathetic, nervous system activation in KM rats. Together, these findings indicate that social load in KM rats evokes a stress response, marked by a hypolocomotion and an atypical autonomic reaction. Conversely, control rats, which displayed a typical fight response to unfamiliar intruders, exhibited marked sympathetic activation during the sociability test. These findings suggest the KM strain may help model specific aspects of ASD-relevant social behavior alongside autonomic dysregulation.