[新知] ScienceDaily-一朝被蛇咬十年怕草繩
Once Bitten, Twice Shy: Temperature Switch Triggers Aversive Memory
ScienceDaily (July 26, 2010) — It is common sense that it's worth learning
what things and situations are harmful to us if we want to have a long and
healthy life. For example, after getting a nasty sunburn, we learn our
lesson, and apply sun cream before going sunbathing next time. The importance
of such learned avoidance strategies is reflected by the fact that even fruit
flies possess them. These tiny flies can learn to associate a particular odor
with a mild electrical shock. Once they learned this association, they steer
clear of the source of that particular odor in future.
Scientists at the Max Planck Institute of Neurobiology now succeeded in
identifying three nerve cells that play a role in the formation of this
complex association. By altering the surrounding temperature, they could
selectively switch certain cells on and off while the insects moved about
freely and learned.
Prevention is better than cure and avoidance strategies often help to save us
from adversity. A child, for example, quickly learns not to touch a hot stove
once it burned its fingers. Avoidance behavior is so essential that even the
comparatively simple brain of the fruit fly excels in it. If, for example, a
fruit fly is presented with a certain odor together with an electric shock,
it quickly learns to avoid this particular odor by moving or flying off in
the opposite direction. Yet, what actually happens in the brain when two such
different stimuli as an odor and an electric shock are linked up with each
other to cause a change in behavior? It was precisely this basic phenomenon
that scientists at the Max Planck Institute for Neurobiology were determined
to track down.
The advantages of the fruit fly
The Max Planck Research Group "Behavioral Genetics," led by Hiromu Tanimoto,
investigates what happens in the brain of the fruit fly when it learns to
avoid something. Given that the fruit fly's brain is nothing short of minute,
one tends to wonder why the scientists investigate this phenomenon in the
fruit fly. There are two good reasons for their choice. First of all, the
brain of this insect is composed of about one hundred thousand nerve cells
and is therefore considerably more straightforward than, say, a human brain
which has about one hundred billion nerve cells. The cells responsible for
avoidance behavior in the fruit fly can therefore be identified much more
readily. What is more, the researchers can use the wide range of genetic
tools that is already available for the fruit fly to activate or deactivate
certain functions of the animal's brain with great precision.
"We used precisely these qualities to our own advantage," Hiromu Tanimoto
explains. The scientists already knew that the connection between an odor and
an electric shock occurs inside the mushroom-body -- a structure in the brain
of the fly that consists of some 2000 nerve cells. It is also clear that the
neurotransmitter dopamine enables the flies to learn to associate a potential
source of danger with a certain odor. However, until now, it was unclear as
to which of the dopaminergic cells are actually responsible for this.
Non-invasive manipulation
"The problem was that we had to determine which of the nerve cells release
dopamine while the flies are moving about and learning to avoid the odor,"
Tanimoto recapitulates on the study. This is precisely what the scientists
have now succeeded in doing. They introduced a "temperature selector" into
dopamine-releasing cells that contact cells of the mushroom-body. This
stowaway gene stimulated the nerve cells in one set of flies to release
dopamine as soon as the room temperature increased slightly. In a second set
of flies, a different gene caused the nerve cells to become deactivated once
room temperature was increased, no matter what stimulus Tanimoto and his
staff presented to the insects. Thus equipped to manipulate, the scientists
could demonstrate that the activity of three dopamine-releasing cells is
essential for a detected odor to be associated with a negative experience.
The decisive role of these three cells was unambiguous: if the cells were
temperature-activated while the flies picked up on an odor, then the insects
learned to avoid the odor -- even without the electric shock that constituted
the negative association with the odor.
"We can now actually examine the function of individual nerve cells in an
active and behaving animal. This opens up new horizons," Yoshinori Aso, who
is visibly delighted with the outcome of his experiment, adds. Step by step,
the scientists now plan to get to the bottom of how experiences are linked to
each other and how behavior modification develops.
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原始網址:(有圖)
http://www.sciencedaily.com/releases/2010/07/100726101201.htm
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