Researchers at The Rockefeller University have shown for the first time in mice how the brain processes signals from pheromones, essential chemicals used by animals to communicate with each other. Reported in the April 16 issue of Cell, the findings provide the first look at the "wiring diagram" of the accessory olfactory system and show that it differs dramatically from the wiring diagram for the main olfactory system, which all mammals, including humans, use to detect smells.
"We have elucidated for the first time the wiring diagram of the accessory olfactory system in mammals, and we have shown that is more complex than the main olfactory system," says senior author Peter Mombaerts, M.D., Ph.D., assistant professor and head of the Laboratory of Vertebrate Developmental Neurogenetics at Rockefeller. "We think that the complex wiring of the accessory olfactory system reflects the need for the brain to recognize blends of molecules, rather than the individual odorant molecules recognized by the main olfactory system."
Scientists think the accessory olfactory system, also known as the vomeronasal system, is involved in animal communication. Neurons located in a structure called the vomeronasal organ project their axons to a specialized part of the olfactory bulb--the accessory olfactory bulb. The olfactory bulb is the first relay station of the olfactory system, where information is collected, integrated and processed. When the vomeronasal organ is removed from animals, they undergo profound changes in mating behavior and aggression.
In the Cell paper, Ivan Rodriguez, Ph.D., and Paul Feinstein, Ph.D., along with Mombaerts used genetic manipulation techniques to determine the wiring diagram of the accessory olfactory system and found pronounced differences from the diagram of the main olfactory system.
First, the researchers found that all the neurons that express the same pheromone receptor project their axons to multiple targets in the accessory olfactory bulb, called glomeruli. There are about 15 glomeruli in all, compared to only two in the main olfactory bulb. Second, the positions of the glomeruli in the accessory olfactory bulb are variable, whereas in the main olfactory bulb the positions are fixed.
"One could argue that the variable positions of the glomeruli in the accessory olfactory bulb may be accounted for by differences among the mice: they are all genetically slightly different," says Mombaerts. "But this variability is also clear between the two accessory olfactory bulbs of the same animal: the left- and the right-hand sides are not symmetrical."
The researchers showed that when the pheromone receptor gene is knocked out, the projection pattern is very different.
"The neurons seem to go all over the place, suggesting that the receptor need to be expressed to make proper connections," explains Mombaerts.
Mombaerts says that "as an encore," they replaced by genetic manipulation the pheromone receptor with an odorant receptor from the main olfactory system. The researchers found that even with an odorant receptor, the neurons projected their axons to another set of numerous glomeruli, in a complex and variable pattern.
"These last two experiments suggest that the pheromone receptor is involved in setting up the wiring diagram, and strangely enough, its function can be substituted by a molecule whose function is totally unrelated," says Mombaerts.
In fact, Mombaerts says, the two receptors share only one thing in common: both belong to a family of molecules called seven transmembrane receptors, meaning that they cross the cell membrane seven times. These receptors receive information from the environment outside the cell and transmit it inside the cell for processing.
Now that Mombaerts has shown that the wiring diagrams of the two olfactory systems are different, the question is why? The main olfactory system is designed to detect a variety of odors, often at extremely low concentrations, and it must make fine distinctions between molecules with similar structure but different smells.
"Perhaps this is the perfect way to detect odors," suggests Mombaerts. "Spread out the detector neurons across a wide area of the nasal epithelium, so that you maximize the chance that a whiff of air stimulates the correct neurons. Then to make the system sensitive enough all these axons from these neurons come together in the glomerulus and connect to the same second-order neurons."
The mammalian accessory olfactory system provides information about chemical signals produced by individuals of the same species. From the research done on insects, Mombaerts says, most pheromones work as a blend or a cocktail.
"It's a collection of chemical stimuli that allows one animal to determine, for example, that the other one is female, that it is at the right part of the cycle to mate and that it hasn't mated yet," explains Mombaerts. "All this information is transmitted by a few chemicals in a complex blend."
Mombaerts thinks that this may be the reason for the more complex wiring diagram in the accessory olfactory system, which is geared toward recognizing patterns rather than recognizing and distinguishing specific molecules.
Another question posed by the wiring diagram is: why is it variable? Scientists know that the accessory olfactory system is very dependent on the experience of the animal. If the VNO is removed from a sexually naive animal--one that has never mated before--the animal experiences severe sexual dysfunction and won't mate. But, if the VNO is removed from an animal that has mated previously, the animal has no problem in its sex life.
"We think perhaps the wiring pattern is variable because it reflects the experiences of the animal," says Mombaerts. "In effect, experience shapes the connections."
Mombaerts suggests that further studies of the accessory olfactory system may show that, for instance, virgin females will have wiring patterns similar to one another but different from those of sexually experienced females.
Mombaerts also thinks that there is a stochastic, or random, element to the asymmetry observed in the glomeruli of the accessory olfactory system, because the left and the right accessory olfactory bulbs are not mirror images. The neurons of the main and the accessory olfactory systems renew or rejuvenate themselves, a process that is unique in sensory systems. Mombaerts suggests that the replacement of neurons may partially account for the stochastic property of the system.
"Pheromonal information bypasses the cortex and goes straight to other brain structures such as the amygdala and the hypothalamus, affecting behavior and regulating hormone production," Mombaerts says. "What really matters is the principles of the connection patterns between the accessory bulb and those other structures."
Mombaerts suggests that the first stage may show this variability, but the connections from the 15 glomeruli to the amygdala and the hypothalamus are perhaps much less variable.
"Genetic methods have been recently developed by others to study the connectivity in the next step of the pathway," he explains. "What really matters is how this map is connected to higher brain centers."
This research was supported primarily by a grant from the March of Dimes Birth Defects Foundation and by the National Institutes of Health, the Human Frontier Science Program Organization, the Swiss National Foundation, the European Molecular Biology Organization, and Bristol-Myers Squibb. Mombaerts was an Alfred P. Sloan, Basil O'Connor, Guggenheim, Irma T. Hirschl, Klingenstein, McKnight, Rita Allen and Searle Scholar or Fellow.
Rockefeller began in 1901 as The Rockefeller Institute for Medical Research, the first U.S. biomedical research center. Rockefeller faculty members have made significant achievements, including the discovery that DNA is the carrier of genetic information and the launching of the scientific field of modern cell biology. The university has ties to 19 Nobel laureates. Thirty-three faculty members are elected members of the U.S. National Academy of Sciences, including the president, Arnold J. Levine, Ph.D.
Materials provided by Rockefeller University. Note: Content may be edited for style and length.
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