Acute stress induces severe neural inflammation and overactivation of glucocorticoid signaling in interleukin-18-deficient mice

Interleukin-18 (IL18) is an inflammatory cytokine that is related to psychiatric disorders such as depression and cognitive impairment. We previously found that IL18 deficiency may cause hippocampal impairment, resulting in depression-like behavioral changes. However, the potential role of IL18 in stressful conditions remains uncertain. In the present study, we examined the effect of IL18 on neural inflammation and stress tolerance during acute stress. Littermate Il18(+/+) and Il18(−/−) mice were exposed to a single restraint stress for 6 h, and all assessments were performed 18 h after the mice were released from the restraint. In Il18(−/−) mice exposed to acute stress, the immobility times in both the forced swim test and tail suspension test were decreased, although no difference was observed in Il18(+/+) mice. Il1β, Il6, and Tnfα expression levels in the hippocampus of stressed Il18(−/−) mice were significantly higher than those in the other groups. Moreover, the numbers of astrocytes and microglia, including those in the active form, were also increased compared with those in other groups. Regarding the molecular mechanism, the HSF5 and TTR genes were specifically expressed in stressed Il18(−/−) mice. As a potential treatment, intracerebral administration of IL18 to Il18(−/−) mice resulted in partial recovery of changes in behavioral assessments. Our results revealed that IL18-deficient mice were more sensitive and had a longer response to acute stress than that in normal mice. In addition, neural inflammation and augmentation of glucocorticoid signals caused by stress were more intense and remained longer in Il18(−/−) mice, resulting in behavioral changes. In conclusion, IL18 might be an indispensable factor that modulates the stress response and maintains balance between neural inflammation and glucocorticoid signaling.