Abstract
Ketamine is a well-characterized N-methyl-D-aspartate receptor (NMDAR) antagonist, although the relevance of this pharmacology to its rapid (within hours of administration) antidepressant actions, which depend on mechanisms convergent with strengthening of excitatory synapses, is unclear. Activation of synaptic NMDARs is necessary for the induction of canonical long-term potentiation (LTP) leading to a sustained expression of increased synaptic strength. We tested the hypothesis that induction of rapid antidepressant effects requires NMDAR activation, by utilizing behavioral pharmacology, western blot quantification of hippocampal synaptoneurosomal protein levels, and ex vivo hippocampal slice electrophysiology in male mice. We found that ketamine exerts an inverted U-shaped dose-response in antidepressant-sensitive behavioral tests, suggesting that an excessive NMDAR inhibition can prevent ketamine’s antidepressant effects. Ketamine’s actions to induce antidepressant-like behavioral effects, up-regulation of hippocampal AMPAR subunits GluA1 and GluA2, as well as metaplasticity measured ex vivo using electrically-stimulated LTP, were abolished by pretreatment with other non-antidepressant NMDAR antagonists, including MK-801 and CPP. Similarly, the antidepressant-like actions of other putative rapid-acting antidepressant drugs (2R,6R)-hydroxynorketamine (ketamine metabolite), MRK-016 (GABAAα5 negative allosteric modulator), and LY341495 (mGlu2/3 receptor antagonist) were blocked by NMDAR inhibition. Ketamine acted synergistically with an NMDAR positive allosteric modulator to exert antidepressant-like behavioral effects and activation of the NMDAR subunit GluN2A was necessary and sufficient for ketamine-like antidepressant-like behavioral effects. We conclude that NMDAR activation is necessary for the beneficial effects of ketamine and other rapid-acting antidepressant compounds. Promoting NMDAR signaling or other approaches that enhance NMDAR-dependent LTP-like synaptic potentiation may be an effective antidepressant strategy.
SIGNIFICANCE:
The anesthetic and antidepressant drug ketamine is well-characterized as an N-methyl-D-aspartate receptor (NMDAR) antagonist; though, the relevance and full impact of this pharmacology to its antidepressant actions is unclear. We found that NMDAR activation is necessary for the beneficial effects of ketamine and several other putative antidepressant compounds. As such, promoting NMDAR signaling, or other approaches that enhance NMDAR-dependent LTP-like synaptic potentiation in vivo may be an effective antidepressant strategy directly, or acting synergistically with other drug or interventional treatments.
Footnotes
TDG has received research funding from Roche Pharmaceuticals, and was a consultant for FSV7 LLC, during the preceding three years. CAZ is listed as a co-inventor on a patent for the use of ketamine in major depression and suicidal ideation. PZ, CAZ and TDG are listed as co-authors in patents and patent applications related to the pharmacology and use of (2R,6R)-HNK in the treatment of depression, anxiety, anhedonia, suicidal ideation and post-traumatic stress disorders. SMT is listed as an inventor on the use of GABA-NAMs for the treatment of depression. All other authors report no conflict of interest.
We thank Ms. Thatchana Rajasekar for her technical assistance in analyzing the novel-object recognition experiments. We also thank Dr. Patrick Morris who characterized the GluN2A PAM, GNE-5729. Research was supported by NIH grant MH107615 and U.S. Department of Veterans Affairs Merit Awards 1I01BX004062 and 101BX003631, and an investigator-initiated research grant from Allergan Pharmaceuticals to TDG, MH086828 to SMT, and a Brain and Research Foundation (NARSAD; # 26826) Young Investigator Grant to PZ.
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