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May 18, 2007
Competition is a cornerstone of community ecology, the study of interactions between different groups of organisms. Interaction between different groups creates what scientists call selective pressure or "survival of the fittest."
But, "survival of the fittest," is not only limited to animals or plants. It turns out that neurons compete during memory formation and that competition is mediated by a protein known as CREB. In a paper published in last week's issue of Science, researchers used CREB to study the competition between nerve cells during memory formation.
"Making memory is not a conscious act," explains Alcino Silva, a lead author on the paper. "Learning triggers a cascade of chemicals in the brain that influence which memories are kept and which are lost."
The researchers knew from earlier studies that the CREB protein had been linked to keeping memories stable, so they suspected that the protein also played a role in channeling memories to brain cells ready for memory storage.
To test their hypothesis the scientists used laboratory mice. They put CREB into a virus so that they could introduce it into an area of the mouse's brain critical to emotional memory. Then, they tried to see if the mouse could remember a cage that it had visited before.
To keep track of which brain cells stored the mouse's memories about the cage, the scientists used a genetic marker that is expressed in neurons showing recent memory activity. When the scientists examined the brains they found that areas with recent activity also had the highest levels of CREB.
"We discovered that the amount of CREB influences whether or not the brain stores a memory," said Silva. "If a cell is low in CREB, it is less likely to keep a memory. If the cell is high in CREB, it is more likely to store the memory." Even more interesting is that fact that the number of neurons showing the genetic marker, does not vary with CREB levels. This suggests that the rules governing neuron selection during memory formation are competitive, and that CREB affects this competitive process. If competition was not a key factor the number of neurons expressing the genetic marker would vary according to CREB levels -- instead the researchers found the same number of neurons recruited for memory formation regardless of the CREB levels.
These results suggest that there may be new ways of preserving memory in people suffering from Alzheimer's or other brain injury. "By artificially manipulating CREB levels among groups of cells, we can determine where the brain stores its memories," he says. "This approach could potentially be used to preserve memory in people suffering from Alzheimer's or other brain injury," allowing scientists to "guide memories into healthy cells and away from sick cells, in dying regions of the brain."
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