Impact of Masticatory Activity and Rehabilitation on Astrocyte Morphology Across the Molecular Layer of the Dentate Gyrus: Insights from the Outer, Medial, and Inner Sublayers and Their Relationship with Spatial Learning and Memory.

Abstract

The dentate gyrus plays a crucial role in learning and spatial memory, particularly in its middle third molecular layer, which receives the primary afferent input via the medial perforant path. Interestingly, changes in masticatory activity are described to affect this region with visible astrogliosis, release of pro-inflammatory cytokines and oxidative stress, affecting synaptic physiology, and cognition. This study aimed to investigate the impact of altered masticatory activity on spatial memory in young Swiss albino mice, correlating these effects with morphological changes in astrocytes. The mice were divided into three groups: Hard diet with pellets (HD), hard diet/soft diet (HD/SD, reduced masticatory activity), and HD/SD/HD (rehabilitated). The Morris water maze test was used to measure escape latency, while three-dimensional microscopic reconstruction methods provided morphometric data on the astrocytes. Hierarchical clustering analysis validated the existence of four morphological subtypes with decreasing complexity (AST1, AST2, AST3, and AST4), in the outer, middle, and inner thirds of the dentate gyrus molecular layer. Changes in masticatory activity affected the number and distribution of astrocytes subtypes excepting AST3 in the middle third layer. Canonical discriminant function analysis indicated that complexity was the variable most influencing cluster formation. Correlation tests between complexity and escape latency for each animal group showed a significant correlation with a large effect size of 60% [Pearson's R: 0.605, p < 0.001] in the HD group in the middle third, which was disrupted by altered masticatory activity. AST3 morphotype in the middle third showed a linear correlation with learning and spatial memory functions in the HD group [Pearson's R: 0.624, p < 0.001] that disappeared with a reduction in masticatory activity, and nor restored by diet rehabilitation. This finding was not observed for inner and outer layers, supporting the contribution of middle third AST3 to learning and spatial memory. Group comparison tests also revealed that diet differentially impacts astrocyte subpopulations on each third of the dentate gyrus molecular layer. Data validate the influence of the masticatory activity on astrocyte complexity and suggest the existence of AST3 association with spatial memory and learning tasks in young female mice. Further research on the underlying mechanisms of these relationships is essential to identify potential therapeutic targets for cognitive disorders and to develop effective interventions to preserve cognitive function.