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Age-related cognitive decline is one of the major aspects that impede successful aging in humans. Environmental factors, such as chronic stress, can accelerate or aggravate cognitive deficits during aging. While there is abundant evidence that chronic stress directly affects cognitive performance, the lasting consequences of stress exposures during vulnerable developmental time windows are largely unknown. This is especially true for the adolescent period, which is critical in terms of physical, sexual, and behavioral maturation. Here we used chronic social stress during adolescence in male mice and investigated the consequences of this treatment on cognitive performance during aging. We observed a substantial impairment of spatial memory, but not other memory domains, 12 months after the end of the stress period. This hippocampus-dependent cognitive dysfunction was supported by concomitant impairment in LTP induction in CA1 neurons in 15-month-old animals. Further, we observed a decrease of hippocampal BDNF mRNA and synaptophysin immunoreactivity, suggesting plasticity and structural alterations in formerly stressed mice. Finally, we identified expression changes of specific neurotransmitter subunits critically involved in learning and memory, specifically the NMDA receptor subunit NR2B. Taken together, our results identify possible molecular mechanisms underlying cognitive impairment during aging, demonstrating the detrimental impact of stress during adolescence on hippocampus-dependent cognitive function in aged mice.

The postnatal development of the mouse is characterised by a period of hypo-responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to moderate stressors. Maternal separation disinhibits this blockade of the HPA axis, but the mechanism responsible is not clear. The present study examined the influence of metabolic signals on the central and peripheral components of the HPA axis in neonatal mice aged 8 days in absence or presence of the mother. Reductions in plasma glucose and leptin as well as rapid increases in plasma ghrelin were apparent in the neonate 4 h following maternal deprivation and maximal at 8 h. In addition, maternal separation induced an increase of neuropeptide Y (NPY) mRNA expression in the arcuate nucleus, a decrease of corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus and a rise in serum corticosterone. Pharmacological manipulation of the metabolic signals attenuated the HPA response to maternal separation. Thus, the rise in plasma corticosterone induced by maternal separation was ameliorated by prevention of reduction in blood glucose or blockade of the ghrelin signalling pathway, as were the hypothalamic changes in NPY and CRH mRNAs. By contrast, leptin treatment did not affect the HPA axis response to maternal separation. Together these results suggest that metabolic signals play an important role in triggering the HPA response of the neonate to maternal separation.

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