Sedentary Life and Reduced Mastication Impair Spatial Learning and Memory and Differentially Affect Dentate Gyrus Astrocyte Subtypes in the Aged Mice.

Fabíola De Carvalho Chaves De Siqueira Mendes,Luisa Taynah Vasconcelos Barbosa Paixão,Daniel Guerreiro Diniz,Daniel Clive Anthony,Marcia Consentino Kronka Sosthenes,Dora Brites,Cristovam Wanderley Picanço Diniz

doi:10.3389/fnins.2021.632216

Abstract

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 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-month-old) 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.

Highlights

The removal of molar teeth has been associated with impaired spatial learning in middle age and aggravation as aging progresses, suggesting that decreased masticatory activity accelerates cognitive decline in aging (Watanabe et al, 2001b; Kondo et al, 2016; Dintica et al, 2020), while its preservation is important for cognitive function (Teixeira et al, 2014; Chen et al, 2015)
We have previously demonstrated that the reduction in masticatory activity by a powder diet on the aged mice previously fed with a hard diet led to spatial learning impairment in the Morris Water Maze (MWM) test (Mendes et al, 2013)
We tested whether the reduction in masticatory activity and the sedentary lifestyle aggravated cognitive decline, and if masticatory rehabilitation and an active lifestyle led to a recovery or delay of cognitive losses trying to track an association with astrocyte morphological changes in the molecular layer of dentate gyrus

Summary

INTRODUCTION

The removal of molar teeth has been associated with impaired spatial learning in middle age and aggravation as aging progresses, suggesting that decreased masticatory activity (chewing) accelerates cognitive decline in aging (Watanabe et al, 2001b; Kondo et al, 2016; Dintica et al, 2020), while its preservation is important for cognitive function (Teixeira et al, 2014; Chen et al, 2015). The learning and spatial memory tested by MWM test (Morris, 1984) require the acquisition of spatial location of relevant extra- or intramaze visual cues that are subsequently processed and retrieved to successfully navigate toward the submerged platform and escape from water (Buccafusco, 2009) This hippocampal-dependent task (Morris et al, 1982; Eichenbaum, 1996; Ergorul and Eichenbaum, 2004) activates the perforant paths and stimulate the outer and middle thirds of mouse dentate gyrus molecular layer (van Groen et al, 2003), which are related to the circuits associated with object identity recognition and spatial memory, respectively (Hunsaker et al, 2007, 2008). We analyzed in detail their morphometry at the outer third of molecular layer given its relevance as an important route of afferents to the hippocampus, and we further quested these findings to the impact on learning and spatial memory of mice

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ETHICS STATEMENT