"Our interest is in the mechanism of action that ketamine uses to be active in the human brain," Dr Muthukumaraswamy said. "That will give us a target for other compounds."
He said ketamine, developed in the 1960s, was off-patent now and primarily used as an anaesthetic and sometimes for chronic pain.
Only low doses of ketamine were needed, compared to anaesthetics, to create those anti-depressant effects, the University of Auckland said,
"We know that ketamine is active in the frontoparietal circuit of the brain and disconnects these two parts," he said.
"In depression those two parts of the brain work overtime in an over-connected way. It may be that ketamine works as an antidepressant by disconnecting those two parts of the brain and stopping that over-connectivity."
He said ketamine, unlike other anti-depressants, was very fast-acting. Yet other clinical trials found the ketamine-induced anti-depression effect only lasted a week or so.
"It's important as a mechanism to identify potential biomarkers of antidepressant activity in human patients, but for other reasons, we cannot use it for treatment."
Dr Muthukumaraswamy said ketamine was not licenced for depression because it was still very experimental in that role.
"Ketamine's anti-depressant properties were discovered relatively recently and did well in clinical trials. Unfortunately, ketamine is also a drug of abuse, as it's mildly hallucinogenic and it is unclear if it could be used in routine clinical practice," he said.
The research team investigated what effect ketamine was having on the brain to work so quickly. Further work is needed to explore this mechanism, he says.
The data was collected by colleagues while Dr Muthukumaraswamy was based at the University of Cardiff. Those colleagues tested the ketamine in healthy volunteers to see what parts of the brain it affects.
The findings were published today in the Journal of Neuroscience.
Dr Muthukumaraswamy is a Rutherford Fellow and a senior research fellow in the university's schools of Pharmacy and Psychology at the University of Auckland. He's also a member of the University's Centre for Brain Research.
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