Scientists have created a potential new treatment for depression by combining a medication for pain relief with one for combating addiction.
Current antidepressants increase the level of serotonin in the brain, though the exact mechanism by which they work is unclear.
Up to 50pc of patients do not respond to the treatment, which can take several weeks to work and cause significant side-effects.
A team at the University of Bath have combined buprenorphine, a painkiller used post-surgery, and naltrexone, used for treating addiction.
The combination produced antidepressant-like responses in mice.
Researchers believe the time it would take to gain regulatory approval for the treatment could be reduced as both drugs are already licensed .
"No new drugs for depression have been developed for decades - they all work in a similar way - so there's an urgent need to develop new treatments," said Dr Sarah Bailey, of the university's Department of Pharmacy & Pharmacology.
Buprenorphine reduces the patient's response to stress by blocking a receptor in the brain, but it also stimulates a related receptor called the mu opioid, which could cause addictive effects if taken long-term.
To counter this, the researchers used the anti-addiction drug naltrexone, which blocks the mu receptor.
They found for the first time that in mice this combination gave an antidepressant effect.