NMDAR inhibition-independent antidepressant actions of ketamine metabolites

Abstract

Major depressive disorder affects around 16 per cent of the world population at some point in their lives. Despite the availability of numerous monoaminergic-based antidepressants, most patients require several weeks, if not months, to respond to these treatments, and many patients never attain sustained remission of their symptoms. The non-competitive, glutamatergic NMDAR (N-methyl-d-aspartate receptor) antagonist (R,S)-ketamine exerts rapid and sustained antidepressant effects after a single dose in patients with depression, but its use is associated with undesirable side effects. Here we show that the metabolism of (R,S)-ketamine to (2S,6S;2R,6R)-hydroxynorketamine (HNK) is essential for its antidepressant effects, and that the (2R,6R)-HNK enantiomer exerts behavioural, electroencephalographic, electrophysiological and cellular antidepressant-related actions in mice. These antidepressant actions are independent of NMDAR inhibition but involve early and sustained activation of AMPARs (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors). We also establish that (2R,6R)-HNK lacks ketamine-related side effects. Our data implicate a novel mechanism underlying the antidepressant properties of (R,S)-ketamine and have relevance for the development of next-generation, rapid-acting antidepressants.