To make memories, new neurons must erase older ones

Nov 12, 2009
neuron
Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons. (Source PLoS Biology). Image via Wikimedia Commons.

Short-term memory may depend in a surprising way on the ability of newly formed neurons to erase older connections. That's the conclusion of a report in the November 13th issue of the journal Cell that provides some of the first evidence in mice and rats that new neurons sprouted in the hippocampus cause the decay of short-term fear memories in that brain region, without an overall memory loss.

The researchers led by Kaoru Inokuchi of The University of Toyama in Japan say the discovery shows a more important role than many would have anticipated for the erasure of memories. They propose that the birth of new promotes the gradual loss of traces from the as those memories are transferred elsewhere in the brain for permanent storage. Although they examined this process only in the context of fear memory, Inokuchi says he "thinks all memories that are initially stored in the hippocampus are influenced by this process."

In effect, the new results suggest that failure of will lead to problems because the brain's short-term memory is literally full. In Inokuchi's words, we may perhaps experience difficulties in acquiring new information because the storage capacity of the hippocampus is "occupied by un-erased old memories."

Of course, Inokuchi added, "our finding does not necessary deny the important role of neurogenesis in memory acquisition." Hippocampal neurogenesis could have a dual role, he says, in both erasing old memories and acquiring new ones.

Earlier studies had shown a crucial role for the hippocampus in memorizing new facts. Studies in people with impaired and normal memories and in animals also showed that information recall initially depends on the hippocampus. That dependence progressively decays over time as memories are transferred to other regions, such as the . Scientists have also observed a similar decay in the strength of connections between neurons of the hippocampus, a phenomenon known as long-term potentiation (LTP) that is considered the cellular basis for learning and memory.

Scientists also knew that new neurons continue to form in the hippocampuses of adults, even into old age. But it wasn't really clear what those newborn brain cells actually do. Inokuchi's team suspected that the integration of new neurons was required to maintain neural connections, but they realized it might also go the other way. The incorporation of new neurons into pre-existing neural circuits might also disturb the structure of pre-existing information, and indeed that is what their new findings now show.

The researchers found that irradiation of rat's brains, which drastically reduces the formation of new neurons, maintains the strength of neural connections in the hippocampus. Likewise, studies of in which hippocampal neurogenesis was suppressed by either physical or genetic means showed a prolonged dependence of fear memories on that brain region.

On the other hand, voluntary exercise, which causes a rise in the birth of new neurons, sped up the decay rate of hippocampus-dependency of memory, without any .

"Enhanced neurogenesis caused by exercise may accelerate memory decay from the hippocampus and at the same time it may facilitate memory transfer to neocortex," Inokuchi said. "Hippocampal capacity of memory storage is limited, but in this way exercise could increase the [brain's overall] capacity."

The study sets the stage for further examination of the connections between neurogenesis and learning capacity, the researchers say. They also plan to examine how the gradual decay of memory dependence on the hippocampus relates to the transformation of memory over time from a detailed and contextually-rich form to a more generic one.

Source: Cell Press (news : web)

Explore further: Top Japan lab dismisses ground-breaking stem cell study

add to favorites email to friend print save as pdf

Related Stories

New research shows why too much memory may be a bad thing

Mar 29, 2007

New research from Columbia University Medical Center may explain why people who are able to easily and accurately recall historical dates or long-ago events, may have a harder time with word recall or remembering the day’s ...

Sleep helps build long-term memories

Jun 24, 2009

(PhysOrg.com) -- Experts have long suspected that part of the process of turning fleeting short-term memories into lasting long-term memories occurs during sleep. Now, researchers at the RIKEN-MIT Center for ...

Stem cells are good for the brain

Jul 15, 2008

For some years, scientists have been speculating over why stem cells exist in the brain, as brain regeneration is limited. A German team of neuroscientists believe these stem cells help keep the brain healthy and active.

Recommended for you

Top Japan lab dismisses ground-breaking stem cell study

Dec 26, 2014

Japan's top research institute on Friday hammered the final nail in the coffin of what was once billed as a ground-breaking stem cell study, dismissing it as flawed and saying the work could have been fabricated.

Research sheds light on what causes cells to divide

Dec 24, 2014

When a rapidly-growing cell divides into two smaller cells, what triggers the split? Is it the size the growing cell eventually reaches? Or is the real trigger the time period over which the cell keeps growing ...

Locking mechanism found for 'scissors' that cut DNA

Dec 24, 2014

Researchers at Johns Hopkins have discovered what keeps an enzyme from becoming overzealous in its clipping of DNA. Since controlled clipping is required for the production of specialized immune system proteins, ...

Scrapie could breach the species barrier

Dec 24, 2014

INRA scientists have shown for the first time that the pathogens responsible for scrapie in small ruminants (prions) have the potential to convert the human prion protein from a healthy state to a pathological ...

Extracting bioactive compounds from marine microalgae

Dec 24, 2014

Microalgae can produce high value health compounds like omega-3s , traditionally sourced from fish. With declining fish stocks, an alternative source is imperative. Published in the Pertanika Journal of Tr ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.