Cell: Stem cell development triggers memory formation

How can brain cells remember new memories without forgetting old ones? Scientists have found the answer. They found that specific neurons in the dentate gyrus region of the brain played a significant role in memory formation, depending on whether the neural stem cells that produced the neurons were older or younger.

The research, to be presented on March 30 in the Cell, links the molecular basis of memory formation to the production of new neurons, a finding that could reveal a new class of drug targets for memory disorders.

The findings also suggest that the imbalance between young and old neurons in the brain may disrupt PTSD and normal memory formation during aging.

In this study, the authors tested two types of memory processes in mice. Pattern separation is the process by which the brain differentiates between similar events, such as remembering different flavors of a Madeleine. Instead, pattern completion is used to evoke detailed memories based on limited clues, such as remembering who was tasting Madeleine cookies at the time.

Pattern separation forms unique new memories based on different experiences; Pattern completion restores memory by detecting similarity. People with brain injuries or trauma may not be able to recall the people they see every day. PTSD patients can't forget terrible events. Impaired pattern dissociation due to the loss of young neurons may facilitate pattern completion, which may highlight the terrible memories of relapse observed in PTSD patients. Neuroscientists have long believed that these two opposing and potentially competing processes occur in different neural circuits. The dentate gyrus, a remarkably malleable structure of the central nervous system, is thought to be involved in the dissociation of patterns and pattern-completion CA3 regions in depression, epilepsy, and horrific brain trauma. Instead, MIT researchers found that neurons in the dentate gyrus may perform pattern separation or pattern completion depending on the age of their cells.

MIT researchers evaluated pattern seperation in mice that learned to distinguish between two similar but different rooms: one safe and one associated with unpleasant foot shocks. To test their pattern completion, the rats were given limited clues to escape the maze they had learned to negotiate. Normal mice were compared with mice that lacked either young or old neurons. Mice showed defects in pattern completion or segregation, depending on which group of neurons was deleted.
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