Scientists have created a potential new treatment for depression by combining a medication for pain relief with one for combating addiction.
Current antidepressants, called SSRIs, increase the level of serotonin in the brain although the exact mechanism by which they work is unclear.
Between 30% and 50% of patients do not respond to the treatment, which can take several weeks to work and cause significant side effects, researchers say.
A team at the University of Bath have combined buprenorphine, a painkiller used post-surgery, and naltrexone, a drug used for treating addiction.
Their combination, which targets at different pathway in the brain to SSRIs, produced antidepressant-like responses in mice.
Researchers believe the time it would take to perform clinical trials and gain regulatory approval for the treatment could be reduced as both drugs are already licensed for other conditions.
Dr Sarah Bailey, senior lecturer at the University of Bath's Department of Pharmacy & Pharmacology, said: "Whilst SSRIs work for a lot of people, they can cause serious side effects and don't work for everyone.
"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 for this condition that affects around four million adults in the UK."
Buprenorphine reduces the patient's response to stress by blocking a receptor in the brain called the kappa opioid.
However it also stimulates a related receptor called the mu opioid, which could cause addictive effects if taken long term or used by depressed patients.
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.
Dr Bailey added: "Our study shows that using a combination of naltrexone and buprenorphine gives an antidepressant effect in mice, but without the problems of addiction that could be caused by using buprenorphine alone.
"Developing new drugs is a lengthy process, with lots of safety tests and trials to go through. These two drugs have already gone through that process individually, so we know they are safe. There still need to be further trials of the combination in humans, but the whole process of developing this as a new treatment would be greatly reduced."
Co-author Professor Steve Husbands added: "It may be hard to deliver this combination in the clinic because of the properties of these drugs. We have also been working on changing the chemistry of buprenorphine so that it has the properties of this combination treatment in one molecule which should simplify drug delivery."
The team has published their findings in the Journal of Psychopharmacology.