Scientists learn how young brains form lifelong memories by studying worms' food choices
a bit of their time watching worms move around. These tiny
creatures, Caenorhabditis elegans, feed on soil
bacteria, and their very lives depend on their ability to
distinguish toxic microbes from nutritious ones. In a recent
study, Bargmann and her colleagues have shown that
worms in their first larval stage can learn what harmful
bacterial strains smell like, and form aversions to those
smells that last into adulthood.
Many animals are capable of making vital, lifelong memories
during a critical period soon after birth. The phenomenon,
known as imprinting, allows newly hatched geese to bond
with their moms, and makes it possible for salmon to return
to their native stream after spawning. And while the
learning processes of humans may be more complex and
subtle, scientists have long known that our brain's ability to
store a memory and maintain it long-term depends on when
and how that memory was acquired.
"In the case of worms, we were fascinated to discover
that their small and simple nervous system is capable of not
only remembering things, but of forming long-term
memories," says Bargmann, who is Torsten N. Wiesel
Professor and head of the Lulu and Anthony Wang
Laboratory of Neural Circuits and Behavior, as well as co-
director of the new Kavli Neural Systems Institute at
Rockefeller. "It invites the question of whether learning
processes that happen during different life stages are
biologically different."
In the study, she and Rockefeller graduate student Xin Jin
let both young and adult worms learn to avoid food smells,
and studied in detail the neural circuits that produced
memories of the experience. Their findings, published in Cell,
clarify which neurons, genes, and molecular pathways
distinguish the two types of memory, providing new vistas
into the neurobiology of learning.
Imprinted aversions last a lifetime
When adult C. elegans worms encounter pathogenic
bacteria they avoid it by moving in the opposite direction,
and they shun similar bacteria for about twenty-four hours.
But their memory soon fades.
Young worms, on the other hand, form more lasting
impressions. The researchers allowed newborn worms to
hatch directly onto a lawn of pathogens, and left them
there for their first twelve hours of life -- the first larval
stage. (The bugs gave the worms intestinal infections, but
didn't kill them.) Then, when the worms encountered the
pathogens again as adults -- three days later -- they fled.
Worms that hadn't been hatched onto poisonous bacteria
found them just as attractive as harmless ones.
Jin found that by silencing specific neurons in the worms,
and repeating the learning assays, she was able to
determine each nerve cell's contribution to the memory
process. The results of her experiments show that the
neural circuits that mediate the two types of learning are
similar, but not identical. Many neurons are needed for both
imprinted and adult learning, but cells called AIB and RIM
are uniquely important for the formation of the imprinted
memory during the larval stage.
A similar picture emerged when the researchers compared
the genes and signaling pathways that are activated when
worms form imprinted versus short-term memories. The two
processes rely on similar molecular components, but some
genes were found to be specifically required for only one
type of learning.
"These findings suggest that early imprinting isn't totally
different from other learning--it's the same system
enhanced with some special features," Bargmann says.
How memories are formed, stored, and retrieved
Several neurological processes are at play when we learn
new things. For example, when a baby songbird learns a
song from an adult bird, a memory of the tutor's
performance must first form and be stored in his brain.
Then, when it's time for the bird to debut with his own
song, that memory must be retrieved to practice and then
perform a vocal behavior.
Because most animals' brains are very complex, it has been
difficult for scientists to study these elements of learning in
detail. By using C. elegans, whose modest brain has only
302 neurons, the researchers were able to shed light on
the neural circuits that drive the formation and retrieval of
a worm's memory--and the two processes turned out to be
neurologically distinct.
"We learned that when worms form an early memory of a
food smell, they use one set of neurons to plant that
memory," Bargmann says, "and later in life, when they
encounter the same smell again, they use a different set of
neurons to pull the memory out."
How memories are stored in the brain -- in what neurons
they reside and what constitutes them at the molecular
level -- remains elusive. But Bargmann says her lab's
findings have laid the groundwork for future research into
stored memory and other open questions.
"The most evocative thing about this work is that it reminds
us that learning isn't some fancy innovation of a complex
brain," she says. "It's a fundamental function that any
nervous system can perform."
0 comments:
Post a Comment