Cell Type-Specific Remodelling of the Female Hippocampus by Reproductive Experience and Age
Background: The hippocampus undergoes extensive cellular remodelling throughout life in response to multiple biological factors that shape its structure and function. Aging represents a fundamental driver of brain changes, with the hippocampus being particularly vulnerable to age-related concerns that contribute to cognitive decline and neurodegenerative disease risk. Reproductive experience, including pregnancy and motherhood, triggers profound hormonal fluctuations and metabolic demands that are increasingly recognized as major modulators of brain plasticity, yet represent an understudied and often paradoxical dimension of neurobiological variation. Although reproductive experience and aging are each known to influence biology across the dorsal and ventral hippocampus, their independent and interactive effects on cellular composition have not been systematically examined using quantitative approaches that can resolve cell-type-specific changes. Methods: We performed cell type deconvolution using Single-cell deconvolution of cell types (SCDC) on bulk RNA-sequencing data from female rat hippocampus, comparing nulliparous and parous females across young/older ages (7 month or 13 month old Sprague-Dawley rats, parous: 30 days or 6 months after giving birth) and dorsal/ventral regions. We harmonized 201 cell type annotations from three single-cell reference datasets into 23 biologically coherent categories utilizing female only data. Three-way ANOVA was used to identify independent and interactive effects. Complementary analyses (random forest, PCA, DESeq2) identified parity-associated transcriptional signatures. Cell-specific functional enrichment was performed by weighted gene set enrichment meta-analysis across multiple pathway databases and metrics. Results: Age emerged as the dominant factor affecting hippocampal cellular composition, significantly altering 6 harmonized cell types, particularly impacting microglia and oligodendrocytes. Regional effects were more extensive, affecting 9 harmonized cell types, while age x region interactions were minimal affecting two harmonized cell types. Critically, parity exerted independent effects on three cell populations: dorsal CA3 pyramidal neurons (p=0.0086-0.010 across two datasets) and SST interneurons (p=0.029) both showed 15-25% decreases, while astrocytes increased (p=0.022). Cell-type-specific pathway analysis revealed distinct molecular mechanisms with enrichment for protein degradation pathways in CA3 pyramidal neurons (GSK3B and BTRC:CUL1-mediated degradation; ubiquitin-dependent degradation of Cyclin D), neuroinflammation pathways in astrocytes, and GABAergic signaling disruption in SST interneurons (signaling receptor activity; GPCR ligand binding). Conclusions: Reproductive experience selectively remodels hippocampal cellular architecture through distinct, cell-type-specific molecular programs that operate independently of age and regional factors. These findings show parity as a critical biological variable requiring consideration in neuroscience and aging research and suggest specific cellular targets for understanding how reproductive history influences brain function.