Effects of soft-diet feeding on synaptic density in the hippocampus and parietal cortex of senescence-accelerated mice.

Some investigators have proposed that extracting of the teeth of rats or mice impairs their acquisition of spatial memory, implying that alterations of the neural networks in the brain result from a reduction of masticatory work. To evaluate numerical alterations of synapses in the cerebral cortex caused by reduced masticatory movements, two strains of the senescence-accelerated mouse, SAMR1 and SAMP8, were fed either a pelleted (hard-diet groups, R1-H and P8-H) or a powdered diet (soft-diet groups, R1-S and P8-S) after weaning. Radioimmunoassay using a monoclonal anti-synaptophysin antibody (SY38) revealed that the synaptophysin content in the whole cortex was significantly lower in P8-H compared with R1-H from 3 months to 12 months of age. The soft-diet feeding reduced the synaptophysin content in the cerebral cortex of both strains after 3 months of age. Immunohistochemistry and electron microscopy on the hippocampal formation and parietal cortex of 6-month-old mice showed that synaptic formation was significantly decreased in these areas in both R1-S and P8-S. The reduction rate of synaptic density due to soft-diet feeding was larger in the hippocampus than in the parietal cortex. The working memory of the four groups was tested at 6 months of age on an eight-arm radial maze. Performance significantly differed between R1-H and P8-H, between R1-H and R1-S, and between P8-H and P8-S. The results indicated that soft-diet feeding after weaning period reduces synaptic formation in the cerebral cortex and impairs the ability of spatial learning in adulthood.