Optical imaging IDs mechanism of long-term memory

Using an optical imaging technique, scientists from the Scripps Research Institute have identified an increase in chemical influx into a specific set of neurons in the common fruit fly that is fundamental to long-term memory. The increase—a memory trace—is a signature component of long-term memory, according to Ron Davis, chair of the Scripps Research Department of Neuroscience, who led the study, which was published in the April 13 issue of The Journal of Neuroscience.

The memory trace in question is an increased influx of calcium into a set of neurons after long-term memory forms in a part of the insect brain known as mushroom bodies, a pair of oversized lobes known to mediate learning and memory, particularly the memories of smell. They have been compared to the hippocampus, a site of memory formation in humans.

Increases in calcium influx also occur with learning in other animal models, Davis said, and it seems highly likely a similar correlation exists in humans.

To measure the changes in the Drosophila neurons, Davis and his colleagues used functional optical imaging, which his laboratory helped pioneer for the study of learning and memory. Using protein sensors that become fluorescent when calcium levels are increased, the team was able to highlight changes in the levels of calcium influx into the mushroom body neurons in response to odor learning. These observed memory traces occur in parallel with behavioral changes.

Interestingly, these memory traces occur only with spaced conditioning—where the insects receive multiple episodes of learning, but with periods of rest between each episode. Spaced conditioning is required for long-term memories to form.

In an earlier study last December, also published in The Journal of Neuroscience, Davis found not only that fruit flies receiving spaced conditioning exhibited a long-term memory trace, but also that their memories lasted between four and seven days. In flies that were given a single episode of learning, memory formation lasted only a day and the long-term memory trace failed to form. These two studies are the newest in a series of six studies on the topic.

"The phenomenon of spaced conditioning is conserved across all species," Davis said. "No one really knows why it's important to long-term memory formation but there appears to be something magical about that rest period during learning."

The National Institutes of Health primarily supported this work.

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Posted by Lee Mather

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