Cognitive and neuroinflammatory consequences of mild repeated stress are exacerbated in aged mice.

Peripheral immune stimulation as well as certain types of psychological stress increases brain levels of inflammatory cytokines such as interleukin-1beta (IL-1beta), IL-6 and tumor necrosis factor alpha (TNFalpha). We have demonstrated that aged mice show greater increases in central inflammatory cytokines, as well as greater cognitive deficits, compared to adults in response to peripheral lipopolysaccharide (LPS) administration. Because aged mice are typically more sensitive to systemic stressors such as LPS, and certain psychological stressors induce physiological responses similar to those that follow LPS, we hypothesized that aged mice would be more sensitive to the physiological and cognitive effects of mild stress than adult mice. Here, adult (3-5 months) and aged (22-23 months) male BALB/c mice were trained in the Morris water maze for 5 days. Mice were then exposed to a mild restraint stress of 30 min before being tested in a working memory version of the water maze over a 3-day period. On day 4 mice were stressed and then killed for collection of blood and brain. In a separate group of animals, mice were killed immediately after one, two or three 30 min restraint sessions and blood was collected for peripheral corticosterone and cytokine protein measurement, and brains were dissected for central cytokine mRNA measurement. Stress disrupted spatial working memory in both adult and aged mice but to a much greater extent in the aged mice. In addition, aged mice showed an increase in stress-induced expression of hippocampal IL-1beta mRNA and MHC class II protein compared to non-stressed controls while expression in adult mice was unaffected by stress. These data show that aged mice are more sensitive to both the cognitive and inflammatory effects of mild stress than are adult mice and suggest a possible role for IL-1beta.