Parity and APOEε4 genotype contribute distinct changes to functional connectivity across the middle-aged brain

Cognition and its underlying neurobiology change throughout the trajectory of aging, with prominent sex differences and influences of sex-specific factors. Research has shown that parity (pregnancy and parenthood) uniquely altered various biomarkers of brain health in middle age depending on presence of Alzheimers disease (AD) risk. The present study builds on prior work by providing a comprehensive view of functional connectivity changes and elucidating how network-level dynamics contribute to cognitive outcomes depending on primiparity and APOEe4 genotype, the top genetic risk factor for late-onset sporadic AD risk. We assessed neural activation in middle-aged wildtype and hAPOEe4 rats that were either nulliparous (0 litters) or primiparous (1 litter). Activation of the immediate early gene zif268 was quantified across 19 brain regions implicated in memory and AD. Primiparous hAPOEe4 rats exhibited widespread reductions in neural activation, particularly in the dorsal striatum, nucleus accumbens, frontal cortex, and retrosplenial cortex. Network analyses further revealed that primiparous wildtype rats had the most cohesive and efficient functional connectivity networks. Notably, the hierarchy of influence of brain regions within the neural network shifted based on parity and hAPOEe4 genotype. Activation of hippocampal new-born neurons in conjunction with subregions of the dorsal striatum, frontal cortex, and retrosplenial cortex dynamically predicted cognitive performance in a parity- and genotype-dependent manner. These findings underscore the lasting impact of reproductive history on brain health and cognitive aging, highlighting the need to consider sex-specific experiences in aging and AD research.