Programmed seasonal brain shrinkage in the common shrew via water loss without cell death

Brain plasticity, the brain’s inherent ability to adapt its structure and function, is crucial for responding to environmental challenges but is usually not linked to a significant change in size. A striking exception to this is Dehnel’s phenomenon, where seasonal reversible brain-size reduction occurs in some small mammals to decrease metabolic demands during resource-scarce winter months. Despite these volumetric changes being well documented, the specific microstructural alterations that facilitate this adaptation remain poorly understood. Our study employed diffusion microstructure imaging (DMI) to explore these changes in common shrews, revealing significant alterations in water diffusion properties such as increased mean diffusivity and decreased fractional anisotropy, leading to decreased water content inside brain cells during winter. These findings confirm that brain-size reduction correlates with a decrease in cell size, as our data indicate no reduction in cell numbers, showcasing a reorganization of brain tissue that supports survival without compromising brain function. These findings extend our understanding of neuronal resilience and may inform future research on regenerative mechanisms, particularly during the spring regrowth phase, offering potential strategies relevant to neurodegenerative disease.