Early Deprivation Impairs Perforant Pathway Connectivity and Contextual Memory in Adolescent Male Mice

Early-life adversity causes reduced hippocampal volume and abnormal hippocampal connectivity and function, with evidence indicating more prominent deficits in males compared to females. Reelin-positive projections from the lateral entorhinal cortex (LEC) to the dorsal hippocampus are essential for encoding contextual and semantic memories in diverse mammalian species, including humans and rodents. However, the impact of early-life adversity on these projections and their contribution to hippocampal-dependent deficits have not been reported. Using a modified limited bedding (LB) mouse model of early adversity that extends the impoverished conditions from birth to postnatal day 25 (P25), we found severe impairment in contextual fear conditioning for adolescent LB male but not LB female mice. Using retrograde tracing, we found that the number of reeling-positive projections from the LEC to the dorsal hippocampus is significantly reduced in LB males but not LB females. Further, the number of projections was highly correlated with deficits in contextual memory and hypomyelination in perforant pathway terminals located in the dorsal hippocampus. Ex vivo high-resolution diffusion magnetic resonance imaging confirmed reduced structural connectivity between the entorhinal cortex and the dorsal hippocampus and revealed extensive cortical atrophy that resembled abnormalities reported in children exposed to severe deprivation. Given the essential role that reelin-positive projections play in contextual memory, these findings suggest a novel mechanism to explain the pronounced contextual memory deficits seen in LB males.