Is Ketamine an Opioid?

Ketamine has been shown to be a potent, fast-acting antidepressant for people with treatment-resistant depression. A recent study has shown that when Ketamine is co-administered with Naltrexone, the antidepressant effect is attenuated. Naltrexone is an opioid receptor antagonist, blocking the effect of opiates. It is typically used to help with opiate and alcohol addiction. This means that the antidepressant effects of ketamine rely upon activation of opioid receptors.

Interestingly, people receiving both ketamine and naltrexone still experience the classic dissociative effects and perceptual changes caused by ketamine. This shows that  the dissociative effects alone are not enough to create the antidepressant effects. Does this mean ketamine is an opioid? If so, what does this mean? Read the full text here and a brief commentary here.