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
|
