A common antibiotic which is used to treat a wide range of diseases, from acne to urinary tract infections, may also help treat post-traumatic stress disorder (PTSD) by suppressing fear memory, suggest results of a trial conducted in a group of health volunteers.

PTSD is a term for a broad range of psychological symptoms that can develop after someone experiences or witnesses a traumatic event.  The disorder is caused by an overactive fear memory, and the new research, published in the journal Molecular Psychiatry, showed that a common antibiotic, doxycycline, can reduce the fear memory response in healthy volunteers. "We have demonstrated a proof-of-principle for an entirely new treatment strategy for PTSD," said lead author Dominik Bach from the University of Zurich in Switzerland.

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," Bach, who is also affiliated to University College London, added.

"Our results support this theory, opening up an exciting avenue of research that might help us to find treatments for PTSD," Bach noted. In the study involving 76 healthy volunteers, participants were given either doxycycline or a placebo and learnt to associate a certain colour with an electric shock. 

The screen would flash either blue or red, and one of the colours was associated with a 50 per cent chance of receiving a painful electric shock.  A week later they were shown the colours again, accompanied by a loud sound but no shocks, and their fear responses were measured.

The fear response was 60 per cent lower in participants who had doxycycline in the first session compared to those who had the placebo, suggesting that the fear memory was significantly suppressed by the drug.  Other cognitive measures including sensory memory and attention were not affected. "When we talk about reducing fear memory, we are not talking about deleting the memory of what actually happened," Bach said. "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," Bach said.

The findings suggest that doxycycline can disrupt the formation of negative associations in the brain.