Mouse Brain Cells Activated, Reactivated in Learning and Memory

"Mouse Brain Cells Activated, Reactivated in Learning and Memory"
http://www.sciencedaily.com/releases/2012/12/121213121345.htm

Humans store episodic memories about events in our lives in the hippocampus, which drives the reactivation of neurons in the cortex. When remembering an event from your life the hippocampus is able to recreate the pattern of cortical activity that was there at the time. Damage to the hippocampus caused patients to lose their memories because they can't recreate the activity in the cortex from when the memory was made. Learning activates a group of neurons that undergo changes, making new connections to store the memory. Retrieving the memory reactivates the exact network used when creating the memory.


To examine what neurons were involved an experiment with mice was set up. The mice were put into a new cage with an unfamiliar odor and given a few minutes to explore. Then they were given a mild electrical shock through the cage floor. When returned to the cage a couple of days later, the mice would remember the shock and stay frozen in one place. When the brains were examined, the researchers could see which cells had been activated initially to form the memory and which were reactivated later to recall it.


Researchers use imaging techniques such as functional magnetic resonance imaging, fMRI, to see which areas of the brain are switched on and off in learning and retrieval but cannot pick out an object as small as a single cell. Wiltgen and University of Virginia graduate student Kaycie Tayler used a genetically modified mouse that carries a gene for a modified green fluorescent protein which causes fluorescence in neurons. This would allow researchers to determine individual cells which were involved in learning and retrieving memories.

About 40 percent of the cells in the hippocampus that were tagged during initial memory formation were reactivated when the memory was recalled. There was also reactivation of cells in parts of the brain cortex associated with place learning and in the amygdala, which is important for emotional memory. There was no evidence of reactivation when the mice were tested in a new environment that they did not remember according to Wiltgen.

In future work, Wiltgen's team plans to examine the role of the hippocampus and other brain regions in forming memories and explore new ways to activate or block memories.