Architectural changes to CA1 pyramidal neurons in adult and aged mice after peripheral immune stimulation

The expression of several inflammatory cytokines that inhibit synaptic plasticity and hippocampal-dependent learning and memory is higher in the brains of aged mice compared to young adults after peripheral injection of lipopolysaccharide (LPS). In this study we investigated whether the exaggerated inflammatory cytokine response in the hippocampus of aged mice after IP injection of LPS is associated with architectural changes to dendrites of pyramidal neurons in the dorsal CA1 hippocampus. Compared to young adults, aged mice had higher basal expression of MHC class II, lower basal expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), and a decrease in total dendritic length in both the basal and apical tree. After IP LPS administration, expression of IL-1beta, IL-6, and TNFalpha mRNA was higher in hippocampus of aged mice compared to young adults whereas NGF and BDNF mRNA was reduced similarly in both age groups. The basal dendritic tree was not affected by LPS in either adult or aged mice 72h after treatment; however, length and branching of the apical tree was reduced by LPS in aged but not adult mice. The present findings indicate that a peripheral infection in the aged can cause a heightened inflammatory cytokine response in the hippocampus and atrophy of hippocampal neurons. Architectural changes to dorsal CA1 hippocampal neurons may contribute to cognitive disorders evident in elderly patients with an infection.