Old memories may get the boot from new brain cells.

A new rodent study shows that newborn neurons destabilize established connections among existing brain cells in the hippocampus, a part of the brain involved in learning and memory. Clearing old memories from the hippocampus makes way for new learning, researchers from Japan suggest in the November 13 Cell.

Other researchers had proposed the idea that neurogenesis, the birth of new neurons, could disrupt existing memories, but the *Cell *paper is the first to show evidence supporting the idea, says Paul Frankland, a neuroscientist at the Hospital for Sick Children in Toronto.

Scientists have known that memories first form in the hippocampus and are later transferred to long-term storage in other parts of the brain. For some amount of time the memory resides both in the hippocampus and elsewhere in the brain. What’s not been known is how, after a few months or years, the memory is gradually cleared from the hippocampus.

Researchers have also debated the role of neurogenesis in learning and memory. The hippocampus is one of only two places in the adult brain where scientists know that new neurons form. On the basis of previous studies, many researchers think new neurons stabilize memory circuits or are somehow otherwise necessary to form new memories.

The new study suggests the opposite: Newborn neurons weaken or disrupt connections that encode old memories in the hippocampus.

Kaoru Inokuchi, a neuroscientist at the University of Toyama in Japan, and his colleagues used radiation and some genetic tricks to block neurogenesis in rats and mice that had been trained to fear getting a mild electric shock when placed in a particular cage. Control animals, with normal neurogenesis, eventually were able to bypass their hippocampi and retrieve the fear memory directly from long-term storage. But animals in which neurogenesis had been blocked still depended on the hippocampus to recall the fear memory, the researchers found.

Running on an exercise wheel, which boosts neurogenesis, also sped the rate at which old memories were cleared from the hippocampus.

But that doesn’t mean new neurons aren’t necessary to teach old brains new tricks, says Inokuchi.

“Our findings do not necessarily deny the important role of neurogenesis in memory acquisition,” Inokuchi says. “Hippocampal neurogenesis could have both of these roles, in erasing old memories and acquiring new memories.”

Essentially, the new neurons may aid formation of new memories by keeping the hippocampus from filling up with old ones.

Frankland adds, “This is about as novel as it gets in the field of neurogenesis and memory. It pretty much represents an entirely new framework that other researchers will chip away at for years to come.”

Image: Hippocampal neuron/NIH

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