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By Lori Stiles - University of Arizona News
Using
flying insects, scientists have discovered the details of
how airborne odors dictate brain activity and behavior.
Although moths have antennae that are a million times more
sensitive to odor than is the human nose, researchers believe
that the basic principles of this model olfactory system
apply to all animals, including humans. They report it in
the March 22 Nature. The collaborating scientists are neurobiologists
Neil Vickers, formerly of the University of Arizona and
now at the University of Utah, Thomas Christensen and John
Hildebrand of the University of Arizona, and Thomas C. Baker
of Iowa State University.
Their
article, "Odour-plume dynamics influence the brain's
olfactory code", describes the long-time collaborators'
multidisciplinary effort that combines behavioral and electrophysiological
methods with chemical ecology for new insight into the complex
functioning of the olfactory system.
Neurobiologists
have long studied how animals detect odors and how these
signals are represented in the brain. But their experiments
are generally carried out in contained, isolated environments,
not the conditions an animal would encounter when sniffing
through its natural environment. And these studies typically
focus on a single aspect of olfaction -- either testing
how different odors affect patterns of activity in the brain
or measuring the behavioral responses of the animal.
The
UA scientists and their colleagues went a step farther by
examining how odors in the environment influence BOTH brain
activity AND animal behavior. Specifically, they studied
how the physical characteristics of natural airborne odors
shape the complex patterns of activity in neurons that encode
different odors in its brain.
"We
focus on behaviorally important olfactory stimuli and seek
to understand how those natural odors are detected and analyzed
as they are encountered in the environment," said John
Hildebrand, Regents professor in the Arizona Research Laboratories
Division of Neurobiology (ARLDN).
"Insects
are telling us a lot about the basic principles underlying
the organization and function of nervous systems,"
said ARLDN Research Scientist Tom Christensen. "And
if we can understand these basic principles in other organisms,
then perhaps we can gain insight into how the human nervous
system responds to sensory stimuli."
Insects
and other animals use odor as a primary means of communicating
with one another and for basic survival skills, like finding
food and places to lay eggs. One of the best-studied odors
is the female sex pheromone recognized by male moths. Scientists
have identified not only the specific chemical components
of the pheromone, but years of previous study have provided
detailed knowledge of how this chemical information is processed
in the brain.
To examine
how a plume of odor triggers complex patterns of activity
in the brain and how this correlates with the moth's behavior,
the researchers devised a method to monitor the olfactory
response of a male moth in flight, as he maneuvered through
a natural plume of female sex pheromone.
When
the scent first hits a moth's antenna, specialized nerve
cells along the antenna recognize the odor and produce small
electrical signals that are then transmitted to other nerve
cells in the animal's brain. Team scientists monitored odor
detection by connecting tiny wires to the antenna. The wires
registered an electric current every time the antenna contacted
odor in the plume. The more odor molecules that hit the
antenna, the larger the measured electrical signal.
The
electrical signals that start in the moth's antenna then
make their way to the brain, where information is processed.
The researchers also monitored how the moth's brain processes
information by inserting tiny recording electrodes in individual
brain cells. The electrodes picked up electrical pulses
each time the brain cells fired.
They
discovered that some of the brain cells were very tightly
synchronized to every pulse of odor. Every time odor hit
the antenna, electrical impulses in antennal cells almost
immediately registered as electrical responses in brain
cells. That is, the brain very closely monitors when odor
is present in the animal's environment and when it is not.
And when the appropriate odor is present, activity in only
the specific group of brain cells that is turned on results
in behavioral changes in the animal, such as a change in
flight direction.
This
may explain why in the absence of pheromone, male moths
simply fly back and forth across a windstream searching
for the odor and make no upwind progress toward the odor
source.
But
each time his antenna gets a whiff of pheromone, stimulated
nerve cells in the antenna send a brief signal to the appropriate
subset of nerve cells in the male's brain. Excited brain
cells then direct the moth to alter his flight path. The
moth continues in the new direction until he again senses
pheromone and the process is repeated. He makes an ever-tighter
zigzag flight pattern as he nears the source of the pheromone,
ultimately homing in on the female moth.
Now
that Christensen and Hildebrand have a fairly good grasp
of how these moths respond to the very specific pheromone
odor, they intend to discover how the moth brain responds
to a much wider variety of environmental odors. By learning
how the moth responds to a great range of different odors,
they will also shed more light on how other animals respond
to the global spectrum of scents.
Ultimately,
people may better comprehend why there is more to smell
than meets the nose.
Related
Links:
http://www.neurobio.arizona.edu/arldn/labs/hildebrand/index.htm
http://www.neurobio.arizona.edu/arldn/people/christensen.html
http://www.biology.utah.edu/People/regfaculty/~vickers/vickers.html
http://www.ent.iastate.edu/dept/faculty/baker.html
Contacts: John Hildebrand ,520-621-6626, jgh@neurobio.arizona.edu
Thomas Christensen , 520-621-6631, tc@neurobio.arizona.edu
Neil Vickers, 801-585-1930, vickers@biology.utah.edu
Thomas Baker, 515-294-1610, tcbaker@iastate.edu
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