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.”