Found on the lining of the nasal cavity, olfactory sensory neurons express odorant receptor molecules that detect odors. But a small subset express, instead, an enzyme called guanylyl cyclase-D (GC-D), which the scientists found are activated by exposure to carbon dioxide. (Conversely, all the cells in the lining of the nasal cavity that were activated by carbon dioxide are the GC-expressing neurons.)

"These findings show that a specialized subsystem has evolved in the mouse to detect carbon dioxide," says Peter Mombaerts, professor and head of the Laboratory of Developmental Biology and Neurogenetics at Rockefeller.

Carbon dioxide makes up about four-hundredths of one percent of the atmosphere. Tests showed that mice can accurately detect amounts of carbon dioxide of at least six-hundredths of a percent, which is just above the average atmospheric level.

Mombaerts cautions that scientists still do not know if the GC-D enzyme is responsible for sniffing out, so to speak, carbon dioxide. "GC-D is a marker for the neurons that have specific carbon dioxide sensitivity," Mombaerts says. "At this time, we have not shown that the marker is mechanistically involved in sensory perception of carbon dioxide." The research does, however, suggest that scientists need to watch for behavioral changes in animals as carbon dioxide levels increase in the atmosphere, since, at some point increased levels may be detectable by animals, Mombaerts says.

"Animals may adapt to this gradual and persistent increase. Alternatively, the change may induce behavioral changes, such as an increase in irritability and aggression or a decrease in fertility," he says. ::Newswise