Dopamine Lets Bad Experiences Linger

Aug. 22 -- FRIDAY, Aug. 21 (HealthDay News) -- Imagine being able to prevent the formation of traumatic memories before they take root in the brain.

That's the promise of a new study on long-term memory formation in rats -- assuming the findings can be applied to humans, that is.

Martin Cammarota, of the Pontifical Catholic University of Rio Grande do Sul, Brazil, with colleagues at the University of Buenos Aires, Argentina, demonstrated in rats that it is possible to disrupt long-term storage of certain memories, assuming that intervention occurs at the right time.

"These investigators have identified specific mechanisms and brain areas, and a definite time frame for conversion of short-term memories into long-term memories, and they are actually able to influence that process," said Dr. Randall Marshall, medical director of clinical research at the pharmaceutical company, Sepracor, which specializes in drugs that act on the central nervous system.

Specifically, Cammarota and his team put rats through what's called an inhibitory avoidance task. In this test, a rat is given an electrical shock through the foot. It is then tested periodically to determine whether it remembered the event. Essentially, if the memory becomes "consolidated" -- that is, written to the brain's "hard drive" -- the animal will hesitate to place its foot on the electrified surface (a time delay called "latency").

In this study, when given a relatively weak electrical shock, the animals tended to quickly forget the experience, placing their feet back on the electrified surface with little hesitation seven days after the initial shock. By contrast, rats given a more powerful shock remembered the experience longer, demonstrating latency up to 14 days later.

The team then attempted to prevent long-term memory formation by interfering with the neurotransmitter dopamine, which is associated with pleasurable and painful stimuli. If a blocker for the dopamine receptor was injected into the hypothalamus of the brain 12 hours after the shock -- but not immediately after the shock or nine hours later -- the rats effectively forgot about the experience: seven and 14 days after the shock, they exhibited little to no latency. Conversely, if a dopamine receptor activator was injected 12 hours following a weak shock, the rats remembered the experience as if they had received a strong shock.