Escitalopram alleviates stress-induced Alzheimer's disease-like tau pathologies and cognitive deficits by reducing hypothalamic-pituitary-adrenal axis reactivity and insulin/GSK-3β signal pathway activity

Chronic stress, a causal factor for depression, can also cause cognitive impairments and tau pathology. However, whether and how the selective serotonin reuptake inhibitor antidepressant escitalopram ameliorates these effects are still unclear. In the present study, rats were subjected to chronic mild unpredictable stress for 8 weeks. Following the initial 4 weeks, the stressed animals were separated into susceptible (depressive) and unsusceptible (resistant) groups based on behavioral tests. Then, escitalopram (10 mg/kg i.p.) was administered for 28 days. Pathophysiological changes were assessed by performing behavioral and biochemical analyses. The results showed that both depressive and resistant rats displayed spatial memory deficits and an accumulation of tau in the hippocampus. Increased levels of corticosterone and insulin and a decreased level of glucocorticoid receptor were found in both depressive and resistant rats. We also found that activity-dependent phosphorylated insulin receptor substrate and glycogen synthase kinase-3β (Ser9 site) were significantly decreased in both depressive and resistant rats. However, other important kinases, such as cyclin-dependent kinase 5 and mitogen-activated protein kinase kinase-1/2, did not change in our study. Furthermore, we found that the mRNA expression of tau was increased in depressive and resistant rats. No significant change in LC3B expression was found. Interestingly, almost all the pathological changes in depressive and resistant rats previously mentioned could be reversed by escitalopram. Our results suggested that escitalopram ameliorates cognitive impairments and selectively attenuates phosphorylated tau accumulation in stressed rats through the regulation of hypothalamic-pituitary-adrenal axis activity and the insulin receptor substrate/glycogen synthase kinase-3β signaling pathway.