|Abstract:||The stress response and stressful stimuli are a significant risk factor for a variety of chronic disease conditions. Stress mediates its effects on chronic disease in part through activation of the IL-1 arm of the immune system, which plays a critical role in behavioral modification during stress. Of the various stress theories, only the allostatic model of stress can completely account for how stress can induce a positive adaptation to a stressor or lead to chronic disease. In this theory, energy state is the primary signal for whether or not a stressor is going to induce beneficial or detrimental adaptation. Here, we show that energy state regulates the IL-1 arm of the neuroimmune system to affect behavioral change in response to allostatic overload. In the first study, mice were provided ad lib access to food or fasted for 24 h to induce low energy levels. 24 h fasting in mice reduces the activity of caspase-1 in whole brain and in the prefrontal cortex, amygdala, hippocampus, and hypothalamus by 35%, 25%, 40%, 40%, and 40% respectively. A 24 h fast also results in a 40% reduction in anxiety-like behavior and a 21% and 31% increase in novel object and object location recognition, respectively, at 24 hours following fasting. ICV administration of the caspase-1 inhibitor biotin-YVAD-cmk reduces caspase-1 activity in the prefrontal cortex and amygdala by 55%, respectively, and results in a 64% reduction in anxiety like behavior. Importantly, when caspase-1 or IL1-R1 knockout (KO) mice are fasted, no fasting-dependent reduction in anxiety-like behavior is observed. These results suggest that low energy levels inhibit inflammasome activation and improve behavioral outcomes following allostatic overload.
Next, we sought to determine if high-energy states induce detrimental effects to cognition in opposition to the effects of low-energy status. We show that acute refeeding for 2 hr with high-fat diet (HFD), but not chow or low-fat diet (LFD) reduces novel object recognition (NOR) and novel location recognition (NLR) memory by 13% and 14% respectively. Additionally, HFD refeeding increases caspase-1 activity in the amygdala by ~100%. Novel object and Novel location recognition performance was negatively correlated with kcal consumption during refeeding, while caspase-1 activity was positively correlated with kcal consumption. Further, genetic deletion of either caspase-1 or IL1R1 eliminated recognition memory impairments due to HFD refeeding. Oxidative stress played a causal role in caspase-1 activation and cognitive dysfunction, as the antioxidant n-acetylcysteine (NAC; 50 mg/kg) improved NOR and NLR performance by 18% and 14% respectively, and reduced caspase-1 in HFD fed animals by 169%. Finally, a free-fatty acid receptor 1 (FFAR1) antagonist, capable of inhibiting palmitic acid induced GPCR activity, improved NOR and NLR by 12% and 14% over vehicle treated HFD fed mice. However, FFAR1 antagonism did not have any effect on caspase-1 activation. Taken together, these data show that acute overconsumption during refeeding causes caspase-1 activation in the amygdala which plays a causal role in the cognitive response to type 2 allostatic overload, and this response is mediated by a mechanism involving oxidative stress developed from excess calorie consumption.
Finally, we wanted to determine if local energy state played a role in the behavioral response to a psychological stressor. We show that 6 hr of chronic experimental stressors can impair memory by 12% in the NOR task and increase activation of caspase-1 by 86% within the amygdala. Memory deficits following stress exposure are dependent on caspsase-1 activation and IL-1β signaling pathways. Interestingly, β-adrenergic antagonist propranolol but not the glucocorticoid antagonist mifepristone was able to ameliorate memory deficits and neuroinflammation following stress. Further, we demonstrate that extracellular adenosine acts as critical danger signal mediating the effects of stress on inflammasome activation and memory impairment. Our results demonstrate a novel pathway originating from the catecholamine response to stress, that leads to adenosine release and subsequent caspase-1 activation and memory impairment. In conclusion, the inflammasome complex operates as a master regulator of the stress response in the amygdala by responding to energy state during stress exposure and allostatic overload. Through this mechanism we have shown that low-energy states induced by fasting produce beneficial behavioral adaptations that lower anxiety-like behaviors and improve cognition, and that high-energy states from overfeeding or psychological stress impair memory.