Around 100 million people around the world take antidepressants like Prozac and Zoloft.
However, few are aware beforehand that antidepressants can worsen anxiety in the first few weeks of use.
Until recently scientists have found the side-effect mysterious.
Now, though, they have identified an anxiety circuit in the brain that responds to serotonin.
The study’s findings help underline the fact that serotonin does not just promote good feelings, despite what many think.
Professor Thomas L. Kash, who led the study, said:
“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.
More generally, this finding gives us a deeper understanding of the brain networks that drive anxiety and fear behavior in mammals.”
Studies have long suggested that serotonin can actually have negative effects on mood.
Younger people seem particularly vulnerable.
Scientists at the University of North Carolina (UNC) School of Medicine have now mapped out the pathway in the brain that is sensitive to serotonin and is linked to anxiety.
The regions of the brain are called the dorsal raphe nucleus (DRN) and the bed nucleus of the stria terminalis (BNST).
After activating these areas in mice using serotonin, the scientists observed anxiety-like behaviours.
Testing Prozac on these mice also showed that it made them more anxious.
Next the researchers hope to find a way of blocking the anxiety-inducing effects of antidepressants.
However, it will first need to be confirmed that human brains operate in a similar way to mice in this regard.
Professor Kash 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.”
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The study was published in the journal Nature (Marcinkiewcz et al., 2016).