A common antibiotic called doxycycline can disrupt the formation of negative thoughts and fears in the brain and may prove useful in treating or preventing post-traumatic stress disorder (PTSD), according to research by British and Swiss scientists.

In a specially designed trial involving 76 healthy volunteers who were given either the drug or a placebo pill, those who were on doxycycline had a 60 percent lower fear response than those who were not.

"We have demonstrated a proof-of-principle for an entirely new treatment strategy for PTSD," said Dominik Bach, a professor at University College London and the University of Zurich, who co-led the research team.

"The theory is based on the recent discovery that our brains need proteins outside nerve cells, called matrix enzymes, to form memories. Matrix enzymes are found throughout the body, and their over-activity is involved in certain immune diseases and cancers. To treat such diseases, we already have clinically approved drugs that block these enzymes, including the antibiotic doxycycline, so we wanted to see if they could help to prevent fear memories from forming in the brain. Our results support this theory, opening up an exciting avenue of research that might help us to find treatments for PTSD," he explained.

The theory is based on the recent discovery that our brains need proteins outside nerve cells, called matrix enzymes, to form memories

In the trial, volunteers were given either doxycycline or a placebo and put in front of a computer. The screen would flash either blue or red, and one of the colours was associated with a 50 percent chance of getting a painful electric shock. After 160 flashes with colours in random order, participants learnt to associate the 'bad' colour with the shock.

A week later, under no medication, the volunteers repeated the experiment. This time there were no electric shocks, but a loud sound played after either colour was shown.

Fear responses were measured by tracking eye blinks, as this is an instinctive response to sudden threats. The fear memory was calculated by subtracting the baseline startle response – to the sound on the 'good' color – from the response to the sound when the 'bad' color was showing.

While the fear response was 60 percent lower in those who had doxycycline in the first session, the researchers found that, importantly, other cognitive measures - including sensory memory and attention - were not affected.

"When we talk about reducing fear memory, we're not talking about deleting the memory of what actually happened," Bach said in a statement about the findings published April 4 in Molecular Psychiatry.

"The participants may not forget that they received a shock when the screen was red, but they 'forget' to be instinctively scared when they next see a red screen. Learning to fear threats is an important ability ... helping us to avoid dangers. (But) over-prediction of threat can cause tremendous suffering and distress in anxiety disorders such as PTSD."

PTSD is caused by an overactive fear memory and includes a broad range of psychological symptoms that can develop after someone goes through a traumatic event.

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