Non-classical NMDA receptor subunit GluN3A in a specialized hippocampal region regulates stress-coping strategies in mice

The hippocampus is a highly complex structure, defined by profound structural, functional, and molecular heterogeneity. The GluN3A subunit, which forms non-classic N-methyl-D-aspartate receptors (NMDARs), is enriched in the hippocampal CA1 subregion and has a poorly understood functional role, especially in stress-related behavioral regulation. Here, we employed chronic social defeat stress (CSDS) and GluN3A knockout (KO) mouse models combined with behavioral, molecular, genetic, pharmacological, and circuit-level analyses to investigate the role of hippocampal GluN3A in stress coping. We found that CSDS selectively reduced GluN3A expression in the intermediate CA1 (CA1i) region. Decreased GluN3A expression in CA1i, from either stress or genetic deletion, promoted passive coping behaviors, whereas overexpression of GluN3A in CA1i reversed the phenotype under both conditions. Mechanistically, consistent with GluN3A enrichment in pyramidal neurons, c-fos staining and fiber photometry revealed that GluN3A was essential for CA1i pyramidal neuronal activation during coping behaviors. Further chemogenetic manipulations demonstrated that activating CA1i neurons facilitated active coping, while inhibition induced passivity. Moreover, local administration of D-serine, an agonist of GluN3A-containing NMDARs, rapidly shifted coping behavior from passive to active by activating CA1i neurons via GluN3A-dependent mechanisms. Circuit tracing revealed that CA1i neurons project to multiple sub-hippocampal and cortico-limbic regions, and GluN3A overexpression selectively enhanced downstream neuronal activity in CA3 and infralimbic cortex, but not in the nucleus accumbens. Together, these findings identify GluN3A as a key molecule linking hippocampal subregional specialization with adaptive stress coping, and suggest that the rapid, resilience-promoting effects of D-serine are also GluN3A-dependent.