Regular physical activity in early life is linked to lifelong cognitive health, whereas the effects of early-life physical inactivity on future cognitive function remain unknown. In this study, we examined the hypothesis that physical inactivity during childhood affects DNA methylation in the hippocampus, which is retained over time and leads to cognitive decline in adulthood. Four-week-old male Wistar rats were divided into two groups: a control group （CON） and a physical inactivity group （IN）． Rats in the IN group were housed in narrow cages with approximately half of the usual floor space to restrict their movement for 8 weeks until 12 weeks of age. After this period, all the rats were housed in standard-sized cages until 20 weeks of age. The object location task （OLT） was used to assess cognitive function at 19 weeks old. The dorsal hippocampus was collected immediately after the activity restriction intervention （12 weeks old） and 8 weeks after the intervention ended （20 weeks old） for analysis of genome-wide DNA methylation levels （RRBS）， RNA-seq, and protein expression levels. The IN group's physical activity decreased to about one-quarter that of the CON group between 4-12 weeks of age. In contrast, there were no significant differences in body weight or food intake throughout the experimental period. The IN group showed poor OLT performance and an increased expression of myelination-related genes and proteins in the hippocampus. However, the expression of these genes was not associated with the changes in DNA methylation induced by early-life physical inactivity. Instead, they are linked to DNA hypomethylation observed only in adulthood. These findings demonstrate that physical inactivity during the young period leads to cognitive decline in adulthood. This may be due to the dysregulation of myelination-related gene expression associated with DNA hypomethylation, a delayed consequence of physical inactivity in childhood.
