A novel biting rod model for assessing aggressive behavior in restraint-stressed rats: Role of 5-HT3 receptor and NF-κB-kynurenine pathways.

Preclinical models are essential for understanding the pathophysiology of intermittent explosive disorder (IED) in rodents. However, current models fail to fully uncover the molecular mechanisms behind restraint stress-induced aggression. We introduced a restrainer combined with a biting rod to measure IED-associated symptoms in stressed rats. Activation of 5-HT3 receptors promotes aggression by increasing pro-inflammatory cytokines and nuclear factor kappa B (NF-κB) activity in the brain. NF-κB, in turn, upregulates indoleamine 2,3-dioxygenase 1 (Ido1), which converts tryptophan (a serotonin precursor) into kynurenine, depleting serotonin levels in the amygdala. We examined the roles of 5-HT3 receptors and Ido1 in driving aggression in restrained-stressed rats. Aggressive behavior of rats was assessed in a restrainer with a biting assembly after ondansetron (5-HT3 antagonist) and minocycline (kynurenine pathway inhibitor) treatments. Pro-inflammatory cytokines and NF-κB levels in the amygdala were measured using ELISA and immunohistochemistry. HPLC quantified kynurenine and tryptophan, while Golgi-Cox staining analyzed dendritic spines. Restraint stress induced extensive biting (aggression), elevated pro-inflammatory cytokines (IL-1β, IL-6, TNF-α), and increased NF-κB expression in the amygdala. Elevated kynurenine and decreased tryptophan levels and dendritic spine density were also noticed. Blocking 5-HT3 receptors and inhibiting kynurenine pathway reduced cytokines, kynurenine, and restored tryptophan and spine density. Compared to existing methods, our model offers a more accurate assessment of restraint stress-induced aggression, incorporating molecular pathways and behavioral measures, enhancing understanding of IED. Moreover, stress-induced 5-HT3 signaling may activate NF-κB and kynurenine pathways in the amygdala, potentially contributing to the development of aggressive behavior in rodents.