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Without long-term memory, none of us would be functional human beings. In order to make sense of the world, our memory employs all sorts of reference points, anchors if you will. For instance, one very important building block is the memory of places. Scientists think that the memory for a given environment is stored in specific neurons in the hippocampus, which is the memory formation center in the brain. These neurons are called place cells. Now, German researchers have reported an incredible feat — they’ve ‘reprogrammed’ such place cells in free-roaming mice.

Research suggests that memories about particular locations come together in place maps. Each map is stable as long as we are in that particular environment, but it reorganizes its activity patterns with different locations, thus leading to a new place map for each environment.

Dr. Andrea Burgalossi of the University of Tübingen and colleagues investigated the mechanisms that underlie the reorganization of place cell activity. Two years ago, the same team of neuroscientists showed that so-called silent or dormant cells could be reactivated by electrical stimulation, thereby becoming active place cells. The researchers now revisited this work and built upon it, toying with new ways to form place cells. Quite strangely, the new work suggests that place cells aren’t nearly as stable as we used to think — in fact, place cells can be reprogrammed.

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The team used a novel method based on juxtacellular recording and stimulation where very fine electrodes as thin as strands of hair measure and induce tiny currents along individual place cells. The researchers performed this procedure on rats freely roaming an arena in the lab. By stimulating individual place cells in the rat’s brain in a different location from where they had originally been active, the activity of the place cells could be reprogrammed. In other words, the cells stopped firing in the original locations and became active in the area where the electrical stimulation occurred.

“We challenged the idea that place cells are stable entities. Even in the same environment, we can reprogram individual neurons by stimulating them at specific places”, says Andrea Burgalossi. “This finding provides insights into the basic mechanisms that lead to the formation of new memories”. I “So far, we have reprogrammed single neurons, and it would be fascinating to find what influence this has on place maps as a whole. We would very much like to know what is the minimum number of cells we need to reprogram in order to modify an actual memory trace in the brain.”

Scientific reference: Maria Diamantaki, Stefano Coletta, Khaled Nasr, Roxana Zeraati, Sohie Laturnus, Philipp Berens, Patricia Preston-Ferrer, Andrea Burgalossi: Manipulating Hippocampal Cell Activity by Single-Cell Stimulation in Freely-Moving Mice. In: Cell Reports (in press) April 3rd, 2018.