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    Scientists 'Rewrite' Bad Memories in Mice

    Manipulation of brain cells can switch negative emotions to positive ones, researchers say


    But the findings, published in the Aug. 28 issue of Nature, would have to be replicated in humans before they could influence clinical practice.

    The investigators set out to discover which brain structures are responsible for linking memories to emotions, using laboratory mice with fiber-optic brain implants. Through a technique called optogenetics, researchers used laser light to stimulate parts of a mouse's brain.

    They focused on two parts of the brain -- the amygdala, which processes emotion, and the dentate gyrus of the hippocampus, where much of a memory's context and detail are stored. Context includes information about where an event took place.

    Male mice first were conditioned with either fearful memories through small electric shocks or rewarding memories through interaction with a female mouse. The neurons associated with this emotional context were located in the dentate gyrus, researchers found.

    Two days later, the mice were placed into a large arena. The researchers recorded which half of the arena the mice naturally preferred, and then began stimulating the dentate gyrus cells with light as the mice wandered around the arena.

    Mice conditioned with fear received memory stimulation whenever they were on the side they naturally preferred, and they soon began avoiding that area. On the other hand, mice with happy memories received stimulation when they wandered into the area they preferred less, and they ended up feeling better about that location and spent more time there, the study found.

    Researchers then tried to reverse the emotional responses stored in the rodents' brains. For male mice with fear conditioning, the researchers light-activated the memory cells involved in the fear memory while the mice spent pleasant time with female mice. For mice that had initially received the reward conditioning, memory cells were activated while they received mild electric shocks.

    The reversal worked, and the mice ended up switching the side of the arena they preferred when their memories were stimulated by light.

    Subsequent attempts to alter emotional memories through manipulation of the amygdala didn't work, suggesting that emotions are hard-wired into individual cells inside the amygdala, the study authors noted.

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