Abstract: The effect of chronic stress on learning and memory functions has been one of hot topics in neuroscience. Hippocampus (HP) and frontal cortex (FC) are key brain regions mediating learning and memory, as well as potential targets of stress reactions. Reduced membrane fluidity may represent an important mechanism underlying a variety of structural and functional alterations of the brain. The aim of the present study was to investigate the effects of chronic stress on spatial learning and memory, as well as membrane fluidity of frontal and hippocampal synaptosomes in rats.
A 21-day chronic stress procedure with multiple stressors (footshock, noise, bondage, bondage plus thermal stimulus) was conducted. After that spontaneous behavior and spatial learning and memory performance were tested using open field and Morris water maze tasks. Frontal and hippocampal synaptosomal membrane fluidity and free calcium concentrations ([Ca2+]i) were also examined. The results demonstrated that spontaneous locomotor activities and exploratory behaviors were significantly decreased in stressed animals. Their spatial learning and memory performance were significantly impaired as well. Furthermore, in chronic stressed animals, the frontal and hippocampal synaptosomal membrane fluidity were markedly reduced whereas the [Ca2+]i were significantly increased as compared to control group.
Current findings suggest that chronic stress results in behavioral changes and impairment of spatial learning and memory, which could be mediated by the changes of synaptosomal membrane fluidity and free calcium concentrations.

Key words: chronic stress, learning and memory, membrane fluidity, hippocampus, frontal cortex, synaptosome