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

Proc Natl Acad Sci U S A. 2019 Mar 12;116(11):5160-5169. doi: 10.1073/pnas.1816071116. Epub 2019 Feb 22.


Preclinical studies indicate that (2R,6R)-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 (2R,6R)-HNK to inhibit NMDARs has not been established. We used a multidisciplinary approach to determine the effects of (2R,6R)-HNK on NMDAR function. Antidepressant-relevant behavioral responses and (2R,6R)-HNK levels in the extracellular compartment of the hippocampus were measured following systemic (2R,6R)-HNK administration in mice. The effects of ketamine, (2R,6R)-HNK, and, in some cases, the (2S,6S)-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 (2R,6R)-HNK exerted antidepressant-related behavioral and cellular responses in mice, the ED50 of (2R,6R)-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 (2R,6R)-HNK dose generated maximal hippocampal extracellular concentrations of ∼8 µM, which were well below concentrations required to inhibit synaptic and extrasynaptic NMDARs in vitro. (2S,6S)-HNK was more potent than (2R,6R)-HNK, but less potent than ketamine at inhibiting NMDARs. These data demonstrate the stereoselectivity of NMDAR inhibition by (2R,6R;2S,6S)-HNK and support the conclusion that direct NMDAR inhibition does not contribute to antidepressant-relevant effects of (2R,6R)-HNK.

Keywords: NMDA receptor; antidepressant; depression; hydroxynorketamine; ketamine.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, N.I.H., Intramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Antidepressive Agents / pharmacology*
  • Behavior, Animal / drug effects
  • Excitatory Postsynaptic Potentials / drug effects
  • Hippocampus / drug effects
  • Hippocampus / metabolism
  • Inhibitory Concentration 50
  • Ketamine / administration & dosage
  • Ketamine / chemistry
  • Ketamine / pharmacology*
  • Male
  • Mice
  • N-Methylaspartate / metabolism
  • Protein Subunits / metabolism
  • Pyramidal Cells / drug effects
  • Pyramidal Cells / metabolism
  • Rats
  • Receptors, N-Methyl-D-Aspartate / metabolism*
  • Xenopus laevis


  • Antidepressive Agents
  • Protein Subunits
  • Receptors, N-Methyl-D-Aspartate
  • N-Methylaspartate
  • Ketamine