Differential effect of repeated lipopolysaccharide treatment and aging on hippocampal function and biomarkers of hippocampal senescence

Aging is associated with low-grade chronic systemic inflammation. Elevated peripheral serum cytokines and chemokines contribute to age-related diseases and correlate with cognitive decline. This study compared the effects of repeated lipopolysaccharide (LPS) treatment in young rats to age-related changes in hippocampal-dependent cognition, synaptic transmission, and transcription. Young (5-7 months) Fischer 344 X Brown Norway hybrid rats were injected intraperitoneally once a week for 6-7 weeks with either LPS (1 mg/kg) or vehicle. Older (14-16 months) rats received a similar injection schedule of vehicle. Older-vehicle animals and young-LPS rats exhibited impaired retention of spatial memory. Examination of the transcriptome of the CA1 and the dentate gyrus indicated that older-vehicle and young-LPS animals exhibited an increase in immune response genes. In contrast to aging, young-LPS animals exhibited an increased expression of genes related to the synapse. Even though young-LPS animals increased the expression of synaptic genes, LPS treatment reduced hippocampal CA3-CA1 total synaptic response and N-methyl-D-aspartate receptor (NMDAR)-mediated component of the synaptic response. Interestingly, the decrease in NMDAR function was not redox-sensitive. This study demonstrates that repeated exposure to LPS has long-term effects on hippocampal synaptic transmission and memory; however, young animals exhibited transcriptional recovery after LPS treatment. Recovery likely results from the acute nature of repeated LPS injections, relative to chronic systemic inflammation observed during aging. RNA-sequencing was performed on the hippocampal subregions CA1 and DG from young-vehicle (n=8), young-LPS (n=8), and old-vehicle (n=6) animals. Three different comparisons were made treatment (young-LPS v young-vehicle), age (aged-vehicle v young-vehicle), and young-LPS v aged-vehicle. The final comparison was performed to determine the similarities/differences between the effect of peripheral inflammation and age on the hippocampal transcriptome.