The work, reported in an advance, online issue of the journal Nature on December 6, 2007, furthers the broad and important goal of elucidating how the neurological system can detect and respond to specific cues in of a sea of potential triggers.
“These results are a really exciting starting place for us to understand how pheromones and the brain can shape behavior,” says team leader Lisa Stowers of the Scripps Research Department of Cell Biology.
Pheromones are chemical cues that are released into the air, secreted from glands, or excreted in urine and picked up by animals of the same species, initiating various social and reproductive behaviors.
“Although the pheromones identified in this research are not produced by humans, the regions of the brain that are tied to behavior are the same for mice and people,” says James F. Battey, Jr., director of the National Institute on Deafness and Other Communication Disorders (NIDCD) of the National Institutes of Health, which provided funding for the study. “Consequently, this research may one day contribute to our understanding of the neural pathways that play a role in human behavior. Much is known about how pheromones work in the insect world, but we know very little about how these chemicals can influence behavior in mammals and other vertebrates.”
The Complex Puzzle of Brain Function
Identifying the chemical pathway of signals that make their way through the neurological system is not easy. One of the challenges for scientists studying brain circuits is that the brain is constantly changing. How a brain detects and then responds to the scent of a particular food, for instance, evolves as the animal learns about that food.
But certain behaviors such as aggression responses between male mice tend to be the same each time they are triggered, suggesting a steady pathway through neurological circuits. So, the Stowers group has focused a research program on understanding the aggression pathway as a general model for brain response.
As a first step in the current study, the group sought to identify specific chemical triggers for aggression in mice, which other researchers had shown involved urine. The Stowers group separated out several classes of chemicals within the urine, then individually swabbed each class onto the backs of castrated mice to determine which could spark an aggressive response by another male. Castrated males lose the ability to elicit aggression on their own, so any such response could be attributed to the added chemicals.
Using this experimental setup, the researchers were able to show specific compounds triggered aggression. Upon examination, the scientists found that these compounds fell into two distinct chemical groups-low molecular weight and high molecular weight proteins.
Particularly intriguing were the high molecular weight compounds, as few high molecular weight compounds exist in urine and none had ever before been shown to act as pheromones. The Stowers group focused on these for the remainder of the study.
Tracing Phermones’ Path
Next, the Stowers lab sought to discover the effect of these high molecular weight compounds on two neurological organs that could potentially convey the pheromone signals to the brain. The first, called the vomeronasal organ (VNO), is located above the roof of the mouth in the nasal cavity. The second is the main olfactory epithelium (MOE), found under the eyeball at the top back portion of the nasal cavity.
Which of these two organs is the main starting point for the aggression pathway is somewhat controversial. Stowers' group had shown in past work that mice genetically altered to lack the VNO did not have aggression responses, suggesting this organ plays a key role, but other researchers had made similar findings with knockout mice lacking the MOE.
To further explore this aspect of signal processing, the Stowers team used an assay of their own design that allows the isolation of individual VNO neurons and MOE neurons and measurement of their firing in response to a given chemical cue. The researchers found that, when exposed to high molecular weight compounds, VNO neurons fired indicating that these are the sensory neurons that mediate aggressive behavior. Moreover, the group was able to provide details about both specific neurons and compounds, and further, identify the subset of VNO neurons that fired in response to four specific high molecular weight proteins acting together.
Stowers adds that while the work elucidates the VNO vs. MOE debate, the current study does not settle it, because the yet-to-be-tested low molecular weight compound class could function via the MOE instead of the VNO. This could make sense because the smaller compounds are more easily volatilized, making it easier for them to reach the MOE, which resides much farther back in the nasal cavity than the VNO.
Interestingly, the four high molecular weight pheromone compounds isolated are from a much larger class of proteins, but an individual mouse only produces four, and the combinations produced differs among individuals. In the past, this four-protein signature was thought to be random, but Stowers says it is possible that different combinations of the proteins could code for different responses.
Keith McKeown | EurekAlert!
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