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Pheromones Create a 'Chemical Image' in the Brain

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Duke Health News 919-660-1306

DENVER, CO -- For the first time, researchers have
eavesdropped on the brains of mice as they go about the normal
behaviors of detecting the subtle chemical signals called
pheromones from other animals.

The researchers have discovered that the animals'
pheromone-processing machinery in the brain forms, in essence,
a specific "pheromonal image" of another animal. Such an
"image" of another animal's sex, identity, social standing and
female reproductive status governs a range of mating, fighting,
maternal-infant bonding and other behaviors.

The scientists said that the specificity they discovered in
the neurons that process pheromonal signals is akin to the
"face neurons" in the visual areas of primate brains that are
specifically triggered by facial features of other animals.

The researchers reported their findings in an article to be
published in the journal Science, and in a talk
delivered at the annual meeting of the American Association for
the Advancement of Science. The researchers, postdoctoral
fellow Minmin Luo, Ph.D., and Howard Hughes Medical Institute
investigator Lawrence Katz,
Ph.D., are both at Duke University Medical Center.

"This was truly opening one of the last 'black boxes' in the
brain," said Katz. "We had no idea what to expect; what these
cells were doing. What I found so exciting is that this was a
sensory-related brain region into which no one has ever stuck
an electrode."

A wide range of mammals, from mice to elephants, possess
such a "sixth sense" for detecting pheromones. In such mammals,
pheromones are initially sensed via a specialized sense organ,
called the vomeronasal organ, in the nasal cavity. The
vomeronasal organ actively pumps samples of pheromones into a
sensory cavity, where they are detected by chemical receptors
similar to those used in taste and smell. These sensory neurons
send connections to a neural structure called the accessory
olfactory bulb, where Katz's team did their recordings. Whether
the same structures persist in humans remains
controversial.

"However, this pheromonal system is not a subset of the
olfactory system," emphasized Katz. "In a way, it is no more
like smell than taste is like smell. The two senses just happen
to reside in the same physical region in mammals, but the
processing pathways don't directly talk to each other in the
brain." In the case of mice, communicating via pheromones is
critical to survival, said Katz.

"Since mice live largely in the dark and don't have very
good vision, they don't use vision to make critical
discriminations among animals," he said. "So, we think our
experiments have revealed the front end of a distinct neural
processing system that enables them to make these kinds of
distinctions."

Previous studies of pheromone-processing in the mammalian
brain consisted largely of recording the activity of sensory
neurons in slices of the vomeronasal organ -- the peripheral
sensory organ -- exposed to such substances as urine, known to
contain pheromones. However, said Katz, such studies could not
explore how the responses of these receptors are integrated in
the animal's brain during the natural functioning of the
pheromone system.

"It's like attempting to record from the flight muscles of a
bird while it's sitting on a perch," he said. "The vomeronasal
organ simply doesn't work unless the animal is conscious and
using it. Unless the vomeronasal organ is actively pumping,
delivering pheromones to receptors, neurons in the brain that
process pheromones cannot be activated. And furthermore, we
don't know what these chemical stimuli actually are, their
concentrations or their behavioral context.

"Only by recording from an animal that is actively
investigating another animal can we hope to study this system
realistically." Also, said Katz, pheromones are not volatile,
and under natural circumstances can only be detected when one
animal contacts another, nuzzling it with its snout.

To measure the neural signals from such behavior, Luo and
Katz used an electrode recording system originally developed by
co-author Michale Fee of Bell Laboratories to record activity
of neurons in the brains of birds as they sang. Basically, the
system consists of three tiny micromotors attached to hair-thin
electrodes that are insinuated into the pheromone-processing
region of the animal's brain. By remote control, the scientists
can retract or extend the electrodes by infinitesimal
increments, seeking out individual neurons to record their
activity during behavior. The electrode system is so small and
light that the animal can move about freely, interacting with
other animals, while still sending signals over a thin wire to
a computerized recorder.

In their experiments, Luo and Katz positioned such
electrodes in the "accessory olfactory bulb" (AOB) -- the brain
structure that processes signals from receptors in the
vomeronasal organ -- of male mice. They then placed other mice
into the home cages of the instrumented mice and recorded the
activity of the AOB neurons as the test animals investigated
the visitors.

Luo and Katz exposed the test mice to different strains of
male and female mice.

"The use of different strains was important, because they
represent individuals that are genetically different from each
other," explained Katz. "So, in exposing the mice to other
strains, we were attempting to simulate the animals' exposure
to other individuals with different genetic backgrounds."

An initial surprise from the experiments, said Katz, was
that some AOB neurons fired most actively as the test animals
explored the other animals' heads.

"Urine has been used in much of this work as the main source
of chemical stimuli, since it is known that animals investigate
the anogenital area extensively," he said. "But we found that
actually the head area must also be a rich source of
pheromones. While it was known that chemical glands were
located there, it was not possible to isolate the tiny amounts
of secretion that they produced."

Analysis of the firing activity of the AOB neurons revealed
that they were highly specific, said Katz.

"We found neurons that are highly selective for another
strain, which means in essence that are selective for other
individuals," he said. "We also found neurons that responded
selectively to a combination of the strain and gender of other
mice.

"But what we did not find, even though we expected to, were
neurons that responded to all members of one gender or another.
We found no neurons that said 'this is a male' or 'this is a
female.""

Thus, said Katz, the pheromonal system may include
specialized neurons that combine to create an overall "chemical
image" of another animal.

"It may be the rodent equivalent of face recognition in
higher primates and humans," said Katz. "In primates there are
neurons called 'face cells' that are selective for features
that we attribute particular importance to, such as the eyes
and the mouth. And like those neurons, the selective pheromonal
neurons we found seem to respond to specific combinations of
features."

Luo and Katz also found that the pheromonal neurons reacted
about ten times slower than olfactory neurons, which might
reflect their function.

"The main olfactory system needs to react quickly, for
example to the whiff of a predator," said Katz. "But one could
argue that the pheromonal system is not designed just to get a
whiff and make a quick yes or no decision. Rather, it may place
a premium on deciphering information about individuals, rather
than simply telling informing that another animal is
present."

Beyond the new insights into the pheromonal system, the
scientists' findings offer general lessons about the chemical
senses, said Katz.

"There has been considerable debate about how the brain uses
information from the vast array of chemical receptors in the
olfactory, taste and pheromonal systems," said Katz. "One view
is that the receptors convey rather general information and
that the brain uses elaborate computational schemes to extract
specific information embedded in the receptor's messages. In
that view, the brain circuitry needs to take into account such
aspects as the timing and coding of the receptor input. The
other view is that receptors can be the front end of 'labeled
lines' whose activation conveys the presence of a specific
stimulus in the environment.

"The responses we're seeing argue for the second view,
because we don't see evidence for broadly responsive neurons
that require timing or other correlational information. They
just seem to signal the presence of a certain quality in the
other animal.

"Also, our conclusion is that this neuronal specificity is
likely to reflect specificity present in the vomeronasal
receptors as well. So, the most likely conclusion is that these
specific responses reflect to a large degree a peripheral
specificity, not a sculpting of the response by local circuits
within the AOB," said Katz.

Now that Luo and Katz have begun to map the "front-end"
pheromonal circuitry, they are tracing the processing of that
information deeper into the brain, seeking to understand how
pheromones trigger behaviors. They are also exploring the
genetic programs that are activated within the brain in the
process of pheromone-triggered learning.

"Particularly fascinating about this system is the
phenomenon of pheromonal imprinting, especially in females,"
said Katz. "They essentially memorize the pheromonal image of a
male that they mate with, forming a long-term imprint. And
what's striking is that only a single exposure can form a
memory that lasts a very long time. Studying such systems, we
believe, could give us important insights into the molecular
basis of memory formation in a mammalian brain."

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