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I am surprised that the piece does not mention last year's finding that the anti depressive effects may be linked to the opioid system:

The linked scientific American piece only mentions the glutamate system.

Does anyone know whether the opioid link ended up being valid?

The short answer is that no one knows. It's likely we won't know for a long time.

As for this study, a side-effect of naltrexone is depression. That would seem to me to make it unsuitable to use in a study with ketamine.

Relevant:

Caution Against Overinterpreting Opiate Receptor Stimulation as Mediating Antidepressant Effects of Ketamine

Like all complex pathologies, it is multifactorial, there is no single pathway