Antidepressant-relevant concentrations of the ketamine metabolite (2R,6R)-hydroxynorketamine do not block NMDA receptor function

Preclinical studies indicate that (2 R ,6 R )-hydroxynorketamine (HNK) is a putative fast-acting antidepressant candidate. Although inhibition of NMDA-type glutamate receptors (NMDARs) is one mechanism proposed to underlie ketamine’s antidepressant and adverse effects, the potency of (2 R ,6 R )-HNK to inhibit NMDARs has not been established. We used a multidisciplinary approach to determine the effects of (2 R ,6 R )-HNK on NMDAR function. Antidepressant-relevant behavioral responses and (2 R ,6 R )-HNK levels in the extracellular compartment of the hippocampus were measured following systemic (2 R ,6 R )-HNK administration in mice. The effects of ketamine, (2 R ,6 R )-HNK, and, in some cases, the (2 S ,6 S )-HNK stereoisomer were evaluated on the following: ( i ) NMDA-induced lethality in mice, ( ii ) NMDAR-mediated field excitatory postsynaptic potentials (fEPSPs) in the CA1 field of mouse hippocampal slices, ( iii ) NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and NMDA-evoked currents in CA1 pyramidal neurons of rat hippocampal slices, and ( iv ) recombinant NMDARs expressed in Xenopus oocytes. While a single i.p. injection of 10 mg/kg (2 R ,6 R )-HNK exerted antidepressant-related behavioral and cellular responses in mice, the ED 50 of (2 R ,6 R )-HNK to prevent NMDA-induced lethality was found to be 228 mg/kg, compared with 6.4 mg/kg for ketamine. The 10 mg/kg (2 R ,6 R )-HNK dose generated maximal hippocampal extracellular concentrations of ∼8 µM, which were well below concentrations required to inhibit synaptic and extrasynaptic NMDARs in vitro. (2 S ,6 S )-HNK was more potent than (2 R ,6 R )-HNK, but less potent than ketamine at inhibiting NMDARs. These data demonstrate the stereoselectivity of NMDAR inhibition by (2 R ,6 R ;2 S ,6 S )-HNK and support the conclusion that direct NMDAR inhibition does not contribute to antidepressant-relevant effects of (2 R ,6 R )-HNK.