Natural Regeneration in the Brain Clears Out Old, Makes Way for New

Sheena A. Josselyn, Ph.D. - brain & behavior research expert on mental illness
Sheena A. Josselyn, Ph.D.

How do we continue to learn and make new memories throughout life? This age-old question is being explored with 21st century technology in the University of Toronto laboratory of 2014 NARSAD Independent Investigator Grantee Sheena A. Josselyn, Ph.D., and 2009 NARSAD Young Investigator Grantee Paul W. Frankland, Ph.D. The husband-and-wife team collaborated on a recent study led by lab member Katherine Akers, Ph.D., that implicates neurogenesis, the creation of new neurons. The study found that neurogenesis in the hippocampus, a region in the brain involved in memory formation, may be helping the brain overwrite old memories with new ones.

The finding, published May 9th in the journal Science, was featured in articles in The New York Times on May 12th and in the May 8th issue of Nature, subsequently reprinted in Scientific American. Also participating in the research were 2013 NARSAD Young Investigator Grantee Anne L. Wheeler, Ph.D., of the Toronto group, and 2001 NARSAD Young Investigator Grantee Tsuyoshi Miyakawa, Ph.D., of Fujita Health University, Japan.

In the study the researchers challenged both adult and infant lab animals with a conditioning task based on their remembering. During infancy, hippocampal neurogenesis levels are high and freshly generated memories tend to be rapidly forgotten, a process called infantile amnesia. In the experiments, increasing neurogenesis after the formation of a memory in adult mice induced forgetting in the adult animals. With the infant animals, decreasing neurogenesis after memory formation slowed the forgetting. The experiments bore out a computational model devised by Foundation Scientific Council member Karl Deisseroth, M.D, Ph.D.

Although seemingly counterintuitive, the disruptive role of these neurons makes some sense, Dr. Josselyn commented in the article in Nature. “More neurons increase the capacity to learn new memories in the future,” she says. “But memory is based on a circuit, so if you add to this circuit, it makes sense that it would disrupt it.” Newly added neurons could have a useful role in clearing old memories and making way for new ones.

Also commenting in Nature, Dr. Deisseroth called the Toronto research “impressive,” noting that it “covered everything from genetic and pharmacological interventions, to behavioral interventions, to cross-species comparisons."

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