A common antibiotic seems to have a strange effect on our memories


An antibiotic used to treat chest infections and sexually transmitted infections has been trialled as a potential treatment for post-traumatic stress disorder. It didn’t work, but the results suggest we may need to rethink our understanding of how memories are formed. The drug, doxycycline, has previously been used to prevent memories being formed in the first place. Dominik Bach at the University of Zurich, Switzerland, and his colleagues theorised that the recall of a previously formed memory could be similarly weakened by doxycycline, as both processes are thought to use similar signalling pathways and proteins in the brain. The team asked 78 men and women to look at a series of triangles coloured either orange, turquoise or violet. Two of the colours came with a nasty surprise – a painful electric shock administered 50 per cent of the time – and these were chosen at random for each person. One week later, half the group took a dose of doxycycline, while the other took a placebo. Both groups were asked to sit at a computer for a few hours and told they could do whatever they wanted to kill time. At one point, a previously painful triangle popped up on the screen for 4 seconds. This was to reactivate the memory of the electric shock associated with the triangle. One week after that, the team repeated the first experiment, but without the electric shocks. Instead, every triangle was paired with a loud sound. If the person was expecting an electric shock in response to a particular colour, they would be more startled by the loud noise, as they were also expecting to feel pain. Bach expected that those who had ingested doxycycline would be less startled compared with those who had taken the placebo, as the drug would have affected their recall of the first experiment. But this didn’t happen, suggesting that memory formation and recall don’t share similar signalling pathways after all. The researchers also looked at “extinction learning”, which is when participants naturally realise during the course of the experiment that the triangles won’t deliver an electric shock any more, making them less startled by the loud sounds in the final experiment. They found that the people who had taken doxycycline didn’t realise this as quickly as those who had been given the placebo. Bach says this effect on learning, even after the drug had left the bloodstream, was “truly unexpected”. The results suggest that the antibiotics may be affecting memory on a genetic level, which would explain its effects on learning even a week after ingestion, he says.

New Scientist, 24 October 2019
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