Opioid receptor system contributes to the acute and sustained antidepressant-like effects, but not the hyperactivity motor effects of ketamine in mice.

In 2000, a subanesthetic dose (0.5 mg/kg i.v.) of the dissociative anesthetic ketamine was reported to have both rapid and robust antidepressant effects in patients diagnosed with major depressive disorder and later, ketamine also was shown to be effective in treatment-resistant depressed patients. However, the mechanisms responsible for ketamine's antidepressant effects remain unclear. In 2018, a clinical study reported that pretreatment with the nonselective opioid antagonist naltrexone attenuated the rapid antidepressant effect of ketamine in depressed patients. The current study investigated the potential role of the opioid receptor system in the acute and sustained antidepressant-like and hyperactive effects of ketamine. Mice were tested in the tail suspension test (TST) and differential-reinforcement-of-low-rate responding (DRL) 72 s task which are behavioral screens for antidepressant-like properties. Additionally, open field locomotor activity also was measured. In all behavioral assays, mice were pretreated with the nonselective opioid receptor antagonist naltrexone or saline prior to ketamine administration. The current study found that ketamine (10 mg/kg) produced acute (30 min) and sustained (24 h) antidepressant-like effects in TST, which were attenuated by pretreatment of 2 mg/kg naltrexone. Ketamine (32 mg/kg) also produced an acute antidepressant-like effect in the DRL 72 s task that was attenuated by pretreatment of 2 mg/kg naltrexone. Finally, ketamine (10 and 32 mg/kg) produced hyperactivity in the open field; however, pretreatment with 2 mg/kg naltrexone failed to block the hyperactivity effects ketamine. These results, along with recent clinical findings, suggest that ketamine's antidepressant effects, but not its hyperactive effects, involve activation of the opioid system.