Scientists have unearthed a possible explanation for why leading anti-depressant drugs known as SSRIs initially make some patients anxious and even suicidal.
Medications such as Prozac and Zolog are designed to increase levels of serotonin in the brain, but American scientists have now linked the neurotransmitter to anxiety in mice.
Researchers from the University of North Carolina found that a mild shock to the paws of mice activated serotonin-producing neurons in an area of the brain involved in mood and depression.
The scientists also discovered that artificially increasing these neurons’ activity made the mice anxious.
One of the next steps for the scientists is to confirm that the same anxiety circuit exists in humans as well.
Professor Thomas L. Kash, senior investigator on the project, said: “It’s logical that it would, since we know SSRIs can induce anxiety in people, and the pathways in these brain regions tend to be very similar in mice and humans.”
Kash added: “The hope is that we’ll be able to identify a drug that inhibits this circuit and that people could take for just the first few weeks of SSRI use to get over that hump.”
The researchers said the findings offer a deeper understanding of the brain networks that drive anxiety and fear behaviour in mammals.
More than 100 million people worldwide take SSRIs to treat depression, anxiety and related disorders.
The NHS describes the drugs as “the first choice medication for depression” because they generally have fewer side effects than many other anti-depressants.
But it adds that the side effects “can be troublesome at first, but they’ll generally improve with time”.
According to the health service, common side effects include feeling agitated, shaky or anxious, feeling or being sick, dizziness, blurred vision, low sex drive, difficult achieving orgasm during sex or masturbation, and in men, difficulty obtaining or maintaining an erection.
The study was published in Nature.