**Serotonin: Scientists unmask the dark side of the 'happy hormone'** 06 Sep 2016

Leading anti-depressant drugs like Prozac boost serotonin, but it also appears to be involved in signalling anxiety within the brain

It is known as one of the “happy hormones” and its discovery ultimately led to the development of what were hailed as depression ‘wonder drugs’ like Prozac.

But, despite being prescribed as a treatment for anxiety, these ‘SSRI’ drugs designed to boost levels of serotonin in the brain had a strange and mysterious side-effect. In some cases, they initially made people feel more anxious or even suicidal.

Now a new study, published in the journal Nature, has found that, contrary to the popular view serotonin only promotes good feelings, it also has a darker side.

Researchers in the US delivered a mild shock to the paws of mice and found this activated neurons that produce serotonin in an area of the brain known to be involved in mood and depression.

Artificially increasing these neurons’ activity also appeared to make the mice anxious. Using sophisticate equipment to monitor the mice’s brains, the scientists, from North Carolina University’s medical school, then mapped what they described as an “essential” serotonin-driven circuit “governing fear and anxiety”. Professor Thomas Kash, one of the researchers, 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 behaviour in mammals.”

According to the NHS website, SSRIs are "usually the first choice medication for depression" because they "generally have fewer side effects".

"These can be troublesome at first, but they'll generally improve with time," it says. It says the "common side effects" of the drugs can include: "Feeling agitated, shaky or anxious; feeling or being sick; dizziness; blurred vision; low sex drive; difficulty achieving orgasm during sex or masturbation; in men, difficulty obtaining or maintaining an erection."

The US researchers said the next step was to find out whether the same serotonin brain circuitry exists in humans. “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,” Professor Kash said. They suggested that existing drugs might be capable of blocking the anxiety-inducing effects of serotonin.

“We're hoping to identify a receptor [in the brain] that is already targeted by established drugs,” Professor Kash said. “One of them might be useful for people as they start taking SSRIs.”

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