RACEMIC KETAMINE AND (S)-KETAMINE ENHANCE GLUTAMATE TRANSMISSION IN THE HIPPOCAMPAL CA3-CA1 PATHWAY

Ketamine ((R,S)-ketamine) is widely used as a dissociative anesthetic and has gained prominence for treating psychiatric disorders, including treatment-resistant depression (TRD). Following FDA approval of (S)-ketamine (Spravato™) in 2019, concerns regarding misuse liability have increased. We recently reported that (S)-ketamine alters dopamine dynamics in the nucleus accumbens (NAc) and modulates dopaminergic–glutamatergic interactions underlying behavioral effects. However, its actions on glutamatergic signaling remain region dependent. By blocking NMDA receptors, (S)-ketamine can decrease glutamate transmission in some brain regions while enhancing it in others. Recent studies further suggest that (2R,6R)-hydroxynorketamine (HNK), a ketamine metabolite with rapid antidepressant properties, increases glutamate activity along the CA3–CA1 Schaffer collateral pathway. This circuit is sensitive to stress and hormonal fluctuations and plays a key role in synaptic plasticity, memory, and emotional regulation, processes disrupted in depressive states. The rapid antidepressant effects of (S)-ketamine are thought to rely on glutamate-driven plasticity. To investigate these mechanisms, we compared the glutamatergic effects of (S)-ketamine, racemic ketamine, and HNK in the CA3–CA1 pathway using fiber photometry. Mice received viral expression of the glutamate biosensor iGluSnFR and the calcium indicator RCaMP1.07 in CA1 followed by optical fiber implantation. We measured spontaneous and electrically evoked glutamate signals. Both racemic ketamine and (S)-ketamine increased tonic glutamate levels and enhanced stimulation-dependent glutamate peak amplitudes in CA1, with stronger effects observed for racemic ketamine, whereas HNK did not significantly alter glutamate dynamics. These findings indicate that ketamine and its (S)-enantiomer enhance hippocampal glutamatergic transmission, providing insight into their antidepressant actions.