Impaired plasticity of neurons in aging. Biochemical, biophysical, and behavioral studies.

Age-related correlation of impaired plasticity of neurons (biochemical and biophysical aspects) and behavioral alterations were investigated in young (3.5 months) and extremely aged (approximately 40 months) female Wistar rats. Age-dependent significant differences in second messenger (cAMP and Ins (1,4,5)P3) concentration and signal transduction via muscarinic and dopaminergic receptors were found. The results point to the specifically impaired coupling between dopamine D1 receptor and GS protein, which underlies normal brain aging. However, cholinergic neurotransmission may be modulated at another level in extremely aged rats. Thus, it appears that the site of affection in coupling of receptor and G protein and/or G protein-dependent signal transduction in aging cannot be generalized. This indicates that alterations in the coupling of signal transduction depend on diverse neurotransmitter receptors with advanced age. The age-dependent alterations in the cAMP and PI signal pathways could be due to changes in the physical properties of the membranes. To support this hypothesis, age-dependent changes in the physical state and the biochemical composition of synaptosomal membranes from the cortex, cerebellum, and striatum were examined by measuring the steady-state fluorescence amisotropy of the membrane probes 1,6-diphenyl-1,3,5-hexatriene (DPH), trimethylammonium-DPH (TMA-DPH), and trimethylammoniumpropyl-DPH (TMAP-DPH). Significant differences in the physical properties of the synaptosomal membranes existed between young and very aged rats, expressed by a higher anisotropy in the 40-month-old rat brain tissue. The changes in the physical properties of the membranes were in line with the determined age-dependent alterations in the chemical composition, e.g., the increase in cholesterol content of the aged membranes.