The Come- and Go Memory

5. May 2010

With an increasing distance, memories of the past vanish. The brain does not reconstruct the memory center hippocampus but expands capacities with new nerve cells. A block keeps old memories alive.

Volatile like a fading memory – this proverb depicts how people imagined our memory a few years ago. The brain stores information and as time passes it gets harder and harder to access it. Only selected bits and pieces reach our long-term memory. Two publications in the renowned professional journal Cell though – published within a few weeks – show that our memories don’t seem to just fade slowly but steadily. Seemingly our brain restores information purposefully, does not replace them with new patterns but builds the according nerve center with new neurons.

New nerve cells – new memories

At the end of last year Kaoru Inokuchi and his team at the Japanese University of Tokoyama showed on rats that in the hippocampus new nerve cells are connected with new information while their brain transfers old information to other brain parts like for example the neocortex. The hippocampus plays a central role in storing sensory impressions and their later retrieval. Earlier observations had proven that the central nervous system prefers to resort to it especially for remembering recent incidents. While in adults new nerve cells in other brain parts hardly grow again it’s different with the hippocampus. Here new cells sprout from neuronal stem cell seeds. Now the researchers show with mice that exactly those neurons growing again account for new memories thus replacing old ones. If you inhibit the nerve cell production by for example radiation or gene technology, the rodents store their memories like for example fear of certain situations a whole lot longer. But as the group of researchers also found out, sports promote – in case of the mice training in a wheel – the genesis of new neuronal storage modules in the hippocampus without the old memories fading out. But those mice did not access this region of the brain any more. “Overall the capacity for stored memories in the hippocampus is limited but sports can increase the total capacity”, states Kaoru Inokuchi.

Forgetting due to RAC

A second, new publication deals also with memories and confirms the Japanese model with another animal model. Yu Zhong from Peking/China and his colleagues went on a search for processes of the metabolism of forgetting. They found a key enzyme named “RAC“ – obviously it’s not ‘easy’ to forget without it. Whenever the researchers blocked the RAC metabolism in flies, the memories of not only unpleasant smells and mild electro shocks remained a lot longer – also the life of the flies was extended from normally several hours to more than one day. But if the Fly brain distributes more RAC, the memories vanished a lot faster and the lab animals quickly fall into the odor trap.

Stem cells against Alzheimer and stroke?

Already in 2007, an American group demonstrated that memory failures can be fixed by using neuronal stem cells at least partly. Although the new neurons have to mature there for several weeks before they actively contribute to an enhanced memory. Nonetheless they might help one day to therapy symptoms of deficiency like with Alzheimer or stroke. The ways of discovery of the material memories are made of are rather winding though, for example people with a high RAC level are rather mentally retarded. And the example of Jill Price shows that a large reservoir for memories is not always of advantage. The American woman with the seemingly perfect memory without forgetting is able to remember every incident in her life. The “Spiegel” (German magazine) quotes her: “I don’t look at my past with a distance. It’s more like I am reliving things again and again and it triggers exactly the same feeling in me. It’s like an endless weird movie which sometimes overwhelms me completely. And there is no stop button.” Perhaps this will change in a few years.

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