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Beware: Reading this may swamp your sea horses

by Rowan Hooper

Reading this column could be an unforgettable way to start the new year.

Why? Because as you peruse this piece you are losing something of your past. Sorry about that. It is probably nothing too important. In fact, I am doing you a favor by helping you erase some old, unwanted, unimportant memories clogging up your neurons.

How do I know? Read on.

It’s all to do with a vital part of the brain called the hippocampus. Five hundred years ago, anatomists who dissected and described the brain were struck by a funny little curved structure they found deep in the temporal lobes on either side of the cerebral cortex. They thought they looked like sea horses — so they named them collectively the hippocampus, the scientific name for sea horse, from the Greek hippos (horse) and kampos (sea monster).

For many years, no one knew what the hippocampus did. Now, though, it has been found to play an important role in the formation of memories. When Alzheimer’s disease strikes, for example, the hippocampus is one of the first parts of the brain to be damaged. So understanding how it works is not only key to knowing how our own autobiography is recorded, but it will also help us deal with the “silver tsunami” of an aging population.

This breakthrough in our comprehension of the hippocampus comes because of new Japanese work that suggests our short-term memory — go back to the beginning if you’ve already forgotten that I was talking about memory — may depend on the ability of newly formed neurons in those parts of the brain to erase older connections.

At least that’s what seems to happen in mice and rats.

Kaoru Inokuchi of the University of Toyama, and colleagues, found that new neurons in the brain cause other memories to decay — but overall memory size remains the same. It’s like when your iPod is full of music, you have to delete some songs in order to put new ones in.

Inokuchi says the discovery shows a more important role than many anticipated for the erasure of memories.

In other words, though we can understand intuitively that memory creation is essential, Inokuchi’s work is showing how we might need to prune away excess memories to make new ones.

His team suggest that the birth of new neurons promotes the gradual loss of memory traces from the hippocampus as those memories are transferred to elsewhere in the brain for permanent storage.

“All memories that are initially stored in the hippocampus are influenced by this process,” said Inokuchi, although his team has at the moment only looked at how “brain-pruning” occurs in the context of fear memory.

What this suggests is that problems with making new neurons might lead to “space” problems — just like when your iPod is full. In effect the brain’s short-term memory may literally be full.

As Inokuchi puts it, we might experience difficulties in acquiring new information because the storage capacity of the hippocampus is “occupied by unerased old memories.”

Scientists have known for some time that new neurons continue to form in the hippocampus of adults, even into old age. But it wasn’t clear what those newborn brain cells actually do.

Inokuchi’s team at the Mitsubishi Kagaku Institute of Life Sciences suspected that 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 findings now show.

In the course of their investigations, the researchers irradiated rats’ brains, which drastically reduced the formation of new neurons. They then found that neural connections in the hippocampus that would have otherwise naturally decayed were instead maintained.

Similar experiments that suppressed neurogenesis in the hippocampus by either physical or genetic means also showed prolonged persistence of fear memories.

On the other hand, voluntary exercise, which causes a rise in the birth of new neurons, speeded up the decay rate of hippocampus-dependent memory, but without any memory loss. Could this be another reason why exercise makes us feel better?

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

Inokuchi’s work might explain a puzzling finding about giving up smoking. Nicotine has the effect of stopping new brain cells forming in the hippocampus, but it also has a stimulatory effect on the brain. So when you stop smoking, you lose that stimulation but still might have problems in the hippocampus as a result of your previous nicotine habit.

There is also the intriguing finding that people with the epsilon-4 variant of the gene APOE, which is the biggest risk factor for getting Alzheimer’s, have smaller hippocampi than others. It is unclear, however, whether the smaller size is a cause or effect of Alzheimer’s.

There is still much to learn about the mysterious sea horses of the brain.

Meanwhile, while writing this column, I have come across “Kaiba,” an anime series by Masaaki Yuasa screened on the Wowow TV channel that’s about a man who is the “king of memories.” Kaiba is the Japanese word for hippocampus. Now, I wonder what I’ve forgotten in learning that?

The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human: How New Biology Explains your Journey through Life).”