Effects of exercise on adolescent and adult hypothalamic and hippocampal neuroinflammation

Adolescence is a period of significant brain plasticity that can be affected by environmental factors, including the degree of physical activity. Here we hypothesized that adolescent rats would be more sensitive to the beneficial metabolic and anti-inflammatory effects of voluntary exercise than adult rats, whose more mature brains have less capacity for plasticity. We tested this by giving adolescent and adult Wistar rats four weeks' voluntary access to running wheels. At the end of this period we assessed metabolic effects, including weight and circulating leptin and ghrelin, as well as performance in a novel object recognition test of memory and central changes in neuronal proliferation, survival, synaptic density, and inflammatory markers in hippocampus. We found exercise reduced fat mass and circulating leptin levels in both adults and adolescents but suppressed total weight gain and lean mass in adults only. Exercise stimulated neuronal proliferation in the suprapyramidal blade of the dentate gyrus in both adults and adolescents without altering the number of mature neurons during this time frame. Exercise also increased dentate microglial numbers in adolescents alone and microglial numbers in this region were inversely correlated with performance in the novel object recognition test. Together these data suggest that adolescent hippocampal microglia are more sensitive to the effects of exercise than those of adults, but this leads to no apparent improvement in recognition memory.