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

Abstract

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.