To explore the impact of reduced mastication and a sedentary lifestyle on spatial
learning and memory in the aged mice, as well as on the morphology of astrocytes
in the molecular layer of dentate gyrus (MolDG), different masticatory regimens
were imposed. Control mice received a pellet-type hard diet, while the reduced
masticatory activity group received a pellet diet followed by a powdered diet, and
the masticatory rehabilitation group received a pellet diet, followed by powder diet
and then a pellet again. To mimic sedentary or active lifestyles, mice were housed in
an impoverished environment of standard cages or in an enriched environment. The
Morris Water Maze (MWM) test showed that masticatory-deprived group, regardless
of environment, was not able to learn and remember the hidden platform location,
but masticatory rehabilitation combined with enriched environment recovered such
disabilities. Microscopic three-dimensional reconstructions of 1,800 glial fibrillary acidic
protein (GFAP)-immunolabeled astrocytes from the external third of the MolDG were
generated using a stereological systematic and random sampling approach. Hierarchical
cluster analysis allowed the characterization into two main groups of astrocytes with
greater and lower morphological complexities, respectively, AST1 and AST2. When
compared to the hard diet group subjected to impoverished environment, deprived
animals maintained in the same environment for 6 months showed remarkable shrinkage
of astrocyte branches. However, the long-term environmental enrichment (18-monthold) applied to the deprived group reversed the shrinkage effect, with significant
increase in the morphological complexity of AST1 and AST2, when in an impoverished
or enriched environment. During housing under enriched environment, complexity of
branches of AST1 and AST2 was reduced by the powder diet (pellet followed by powder regimes) in young but not in old mice, where it was reversed by pellet diet (pellet
followed by powder and pellet regime again). The same was not true for mice housed
under impoverished environment. Interestingly, we were unable to find any correlation
between MWM data and astrocyte morphological changes. Our findings indicate that
both young and aged mice subjected to environmental enrichment, and under normal
or rehabilitated masticatory activity, preserve spatial learning and memory. Nonetheless,
data suggest that an impoverished environment and reduced mastication synergize to
aggravate age-related cognitive decline; however, the association with morphological
diversity of AST1 and AST2 at the MolDG requires further investigation.
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