Dopamine is a neurotransmitter that is very similar to adrenaline and associated with pleasurable and painful stimuli. While people induce drugs or smoke cigarettes to stimulate dopamine production to feel good, scientists have found that in rats, dopamine is also key to keeping terrifying memories in the brain's "hard drive" for a much longer time.

Researchers have tested the effect of dopamine on rats by electrocution through their feet. While this brain chemical had no effect on memory if given right after the shock, the rats showed a hesitation to place its foot on the electrified surface for a longer period of time if given a dopamine-enhancing chemical 12 hours later - roughly the time it takes for the brain to consolidate long-term memories. The opposite reaction is recorded when the rats were given a dopamine-reducing drug, the rats tend to forget the painful experience and walked right onto the foot-shocking device again.

Philip Corlett, of the University of Cambridge and Yale University said: "I think it's a really exciting study ... Understanding more about how long-term memories are formed may have huge implications for psychiatric diseases, which often are diseases of memory."

However, further studies have to be done to test this theory on humans.

Source: US News