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Scientists can now turn bad memories into good memories in mice

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What if you could take your bad memories and associate them with more pleasant feelings? That's exactly what a team of scientists have done at MIT, by manipulating a switch in the brain that associates emotions with particular memories in mice.

The research team began with the idea that a specific part of the brain is responsible for linking certain emotions with particular memories in the brain. They found a specific circuit that connects the brain's hippocampus, the part of the brain responsible for memory formation, and the amygdala, the part of the brain which processes emotions.

Every memory has a specific emotion attached to it. For example, remembering a beautiful vacation day at the beach brings happy emotions or remembering an argument brings angry emotions. However, these memories are very flexible, although we don't currently know why. This study sought out an answer to that.

The MIT team genetically engineered mice so that they could tag their hippocampal cells that turn on when memories form with a protein that reacts to light.

Researchers gave half of the mice positive experiences (for example, male mice socialized with female mice), with the other half receiving negative experiences (mild electric shocks). These experiences activated the neurons associated with memory.

The mice were then placed in a large box. Scientists pointed laser light at one side of the box. This light activated the light-sensitive proteins in the rodents' brains. Those mice that received shocks started avoiding that part of the box. The mice with the pleasant experiences, however, showed preference to that side of the box. This suggests that the light was activating the regions of the brain associated with memory emotion.

"The assumption here is that memories are formed between neurons that are active at the same time," says Roger Redondo, co-author of the study. "So if this is right, we should be forcing the neurons associated with fear to begin with to link up with the new neurons expressing the pleasure of spending time with a female."

Scientists then reactivated the memory neurons in the mice that originally received shocks, this time with a pleasant experience: the male mice spent time with female mice. Those mice that originally had positive experiences received electric shocks. When placed back in the box, after exposure to the laser light, the mice who'd once been afraid of one side of the box showed a preference for that side, while the other mice were now fearful of it.

This suggests that the scientific team found the switch that can turn bad memories into good and vice versa.

Scientists hope that their research aids in developing new drugs for humans who suffer from depression, post-traumatic stress disorder and other mental illnesses.

"This is a tour de force of modern molecular-biology-based methods for analyzing processes, such as learning and memory, at the neural-circuitry level," says David Anderson, a professor of biology at the California Institute of Technology. "It's one of the most sophisticated studies of this type that I've seen."

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