The hippocampus as a central hub in ketamine's antidepressant action: from molecules to circuit rewiring.

Ketamine has emerged as a rapid-acting antidepressant that challenges classical monoaminergic frameworks and highlights the importance of synaptic and circuit-level plasticity in mood regulation. This review examines the hippocampus as a key site through which ketamine exerts both rapid and sustained antidepressant effects. We synthesize evidence showing that ketamine enhances hippocampal synaptic plasticity via mechanisms including NMDAR blockade of spontaneous neurotransmission, BDNF-TrkB signaling, MeCP2-dependent transcriptional priming, and adult neurogenesis. Molecular modulators such as Reelin, which influence NMDAR signaling and synaptic function, may also shape the efficacy of ketamine in a subset of individuals. Importantly, these hippocampal effects occur in coordination with broader network interactions, particularly with the medial prefrontal cortex and lateral habenula, allowing for circuit-level integration of antidepressant responses. Notably, ketamine's therapeutic actions are dissociable from normalization of hypothalamic-pituitary-adrenal (HPA) axis function, underscoring a shift away from neuroendocrine-based models. By integrating molecular, synaptic, and systems-level findings, this review provides a hippocampus-centered framework for understanding ketamine's antidepressant mechanisms and outlines novel strategies for circuit-informed, fast-acting antidepressant development.