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Neurons that lead to memory pinpointed

The discovery moves researchers a step closer to understanding how information is learned and remembered.

September 01, 2007|Denise Gellene | Times Staff Writer

Tracing the circuitry of memory in the brain, scientists have found that neurons activated during traumatic experiences also store the memory of those events.

The discovery, reported this week in the journal Science, moved researchers a step closer to understanding how information is learned and remembered -- a scientific journey that could lead to better treatments for people with impaired memories.

Scientists at Scripps Research Institute in La Jolla tracked brain activity in genetically engineered mice whose neurons turned on a specific gene, known as LAC, when a memory was formed, and a second gene, ZIF, when the memory was recalled.

Researchers placed mice in a lemon-scented chamber and subjected them to mild electrical shocks when a tone sounded. The fear-conditioning exercise was repeated three times for each mouse.

Three days later, researchers returned the mice to the fragrant chamber and sounded the tone. The mice froze in fear, a sign they remembered the shocks. Another set of mice were fear-conditioned, but weren't put back into the chamber to recall the experience.

The mice were killed and their brains dissected. Scientists focused on cells in the amygdala, the area of the brain that registers fear and emotion.

Only the mice that were forced to recall their bad experiences had neurons with both genes turned on.

Mice with the strongest memories -- those that displayed the greatest fear -- had the most neurons with both genes activated.

Michael S. Fanselow, a UCLA psychologist who studies memory and was not involved in the study, called the findings important.

"We've never before been able to look at a single neuron that was active during learning and memory retrieval," he said. "And they were able to show the number of neurons is important in terms of the strength of the memory."

Senior author Mark Mayford said the next step was to determine the number of neurons an individual memory required.

"How many can I turn off before I degrade a memory?" he said. "How many do I need to get recognition, and in what order do I turn them on?"

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