Intra-uterine position modulates lifelong hippocampal neurogenesis and anxiety-related behavior
Individual differences in vulnerability to disease are a defining feature of biology and are especially prominent in mood disorders, where susceptibility to stress varies markedly across individuals. Strikingly, such individuality also emerges in inbred mice despite genetic homogeneity and controlled environments, suggesting that early non-genomic factors can durably shape behavioral trajectories. Yet the developmental origin of this variability remains largely unknown. Here, we identify the intra-uterine position (IUP) as a potent source of inter-individual variability in emotional behavior by showing that it influences prenatal steroid exposure, hippocampal development, adult neurogenesis, and anxiety-like traits in inbred mice. At embryonic day 18.5, embryos adjacent to one or two male siblings (i.e. 1M/2M) exhibited altered brain steroid profiles, coinciding with increased proliferation of neural stem and progenitor cells in the dentate gyrus, as compared to 0M mice. Single-nucleus RNA sequencing further revealed that IUP leaves a transcriptional imprint on hippocampal development, with 1M/2M neural stem cells showing reduced stemness and activation of a neurogenic transcription program. In adulthood, 1M/2M mice presented reduced hippocampal and dentate gyrus volumes, diminished radial-glia-like cell counts, lower cell proliferation, and impaired dendritic maturation of immature neurons. Behaviorally, both juvenile (PND22) and young adult (8-week) 1M/2M mice displayed increased trait anxiety, and 1M/2M males exhibited heightened stress reactivity. In vitro, testosterone, but not progesterone, directly enhanced proliferation of adult rat hippocampal stem/progenitor cells, whereas corticosterone reduced it. In vivo, administration of the androgen receptor agonist dihydrotestosterone (DHT) to pregnant dams recapitulated IUP effects by increasing embryonic dentate gyrus (DG) proliferation in female embryos, whereas antagonism reduced progenitor proliferation in male embryos. Together, these results indicate that IUP has long-lasting effects on hippocampal development and anxiety and may underlie natural variations in trait anxiety.