Animals: C57BL/6 (Jackson Laboratory), C57BL/6 aged mice (National Institutes of Aging), Dcx-Luc20 (link), and C57BL/6J-Act-GFP (Jackson Laboratory). All animal use was in accordance with institutional guidelines approved by the VA Palo Alto Committee on Animal Research. Parabiosis surgery followed previously described procedures19 (link) with the addition that peritonea between animals were surgically connected. Immunohistochemistry followed standard published techniques24 (link). Extracellular electrophysiology was performed as previously described25 (link). Spatial learning and memory was assayed with the RAWM paradigm as previously published26 (link). Contextual fear conditioning was assayed as previously published27 (link). Relative plasma concentrations of cytokines and signaling molecules in mice and humans were measured using antibody-based multiplex immunoassays at Rules Based Medicine, Inc. Statistical analysis was performed with Prism 5.0 software (GraphPad Software). Plasma protein correlations were analyzed with the Significance Analysis of Microarray software (SAM 3.00 algorithm; http://www.stat.stanford.edu/~tibs/SAM/index.htm). Experiments were carried out by investigators blinded to the treatment of animals.