Why do we remember bad things? A single chemical may make all the difference

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With a single well-timed injection, scientists show they can erase a bad memory from the mind of a rat.

For their remarkable finding, researchers from Brazil and Argentina gave electric shocks to rats and then tested how long the animals remembered and tried to avoid shocks in the future. The researchers showed that rats will quickly forget a shock if, 12 hours after the event, they’re given a memory-wiping shot of something called a D1 dopamine receptor antagonist — a substance that knocks out major receptors for one of the brain’s key neurotransmitters, dopamine. Rats that received a milder shock, however, could be made to remember that less serious event for longer, if they received a shot of a dopamine promoter 12 hours after the event.

The magic number was 12. Shots didn’t seem to work if given immediately after the fearful event, and they didn’t seem to work if given nine hours after the event. This result, published today in the journal Science, suggests that dopamine may not play much of a role in forming thoughts and fears, but rather in lodging those experiences in long-term memory. And that’s a chemical mechanism that may well occur in humans as well as rats.