Protracted Effects of Ketamine Require Immediate Kappa Opioid Receptor Activation and Long‐Lasting Desensitization

Abstract

Ketamine produces rapid and sustained antidepressant activity, but its mechanism of action remains uncertain. Williams & colleagues (2018) reported that naltrexone pretreatment attenuated ketamine’s effects in treatment resistant depressed patients. Naltrexone antagonizes both kappa (KOR) and mu opioid receptors (MORs). We hypothesized that ketamine requires acute KOR activation, leading to long‐lasting suppression of KOR signaling to exert its antidepressant effects. The following series of experiments demonstrated: 1) exposure to ketamine produces internalization of KORs; 2) the behavioral effects of ketamine in mice were prevented by prior KOR blockade; and 3) ketamine pretreatment prevented behavioral and physiological responses caused by subsequent KOR activation.KOR internalization was tested in HEK293 cells transfected with yPET‐OPRK1 plasmids following ketamine stimulation (2 μM). Approximately 30% of KORs were internalized, which was comparable to the internalization achieved with the endogenous KOR agonist dynorphin (40 μM). To demonstrate that activation of KORs by ketamine contributes to its protracted behavioral profile, C57BL/6J mice (8–12 weeks) were pretreated with naltrexone (1 mg/kg) or the selective KOR antagonist LY2444296 (3 mg/kg) 30 minutes prior to ketamine (10 mg/kg) and were tested in the forced swim test 24 h later. Reductions in immobility scores by ketamine were blocked by naltrexone and LY2444296 pretreatment, confirming the importance of KORs in initiating ketamine’s long‐lasting effects.Next, the physiological impact of ketamine on KOR activity was examined ex vivo in the lateral habenula (LHb), a reward‐related brain region. Bath application of the KOR agonist U50,488 (10 μM) robustly increased the number of action potentials in response to depolarization. The increase in LHb neuronal excitability was absent when ketamine (10 mg/kg) was administered to mice 24 h previously. In parallel, U50,488 (10 or 20 mg/kg) induced behavioral deficits on nest building, prepulse inhibition and antinociception on the hot plate were effectively prevented by ketamine (10 or 20 mg/kg) given 24 h earlier. Finally, LY2444296 (3 mg/kg) administered 30 min prior to ketamine (10 mg/kg) prevented ketamine‐induced suppression of U50,488‐induced alterations on nesting behavior 24 h later. This established that KOR occupation prior to ketamine prevented the initiation of ketamine’s long‐lasting behavioral effects.Collectively, these studies provide evidence supporting the hypothesis that activation of KORs by acute ketamine administration leads to enduring reductions of KOR signaling, which may contribute to the protracted clinical antidepressant effects of ketamine.Support or Funding InformationThis work was funded by US Public Health Service (USPHS) grant R01 MH105623.