Updated publication reference for PubMed record(s): 29958836, 30026405. Aging is believed to be the result of alterations of protein expression and accumulation of changes in biomolecules. Although there are numerous reports demonstrating changes in protein expression in brain during aging, only few of them describe global changes in the protein level. Here, we present a deepest quantitative proteomic analysis of three brain regions, hippocampus, cortex and cerebellum, in mice aged 1 and 12 months, using the total protein approach technique. In all the brain regions, both in young and in middle-aged animals, we identified over 6,700 proteins. We found that although the total protein expression in middle-aged brain structures is practically unaffected by aging, there are significant differences between young adult and middle-aged mice in the expression of some receptors and signaling cascade proteins proven to be significant for learning and memory formation. Our analysis demonstrates that hippocampus is the most unstable structure during natural aging and that the first symptoms of weakening of neuronal plasticity may be observed on protein level in middle-aged animals.