Although invertebrates are known to sense and show behavioral responses to concentrations of CO2 similar to those in Earth's atmosphere, it has been unclear whether the mammalian olfactory system also can sense such amounts of CO2. Hu et al. describe a set of olfactory neurons that appear to allow detection of concentrations of CO2 about 70% greater than those in air. The neurons express carbonic anhydrase II, which catabolizes CO2 and appears to be required as part of the sensing mechanism.
J. Hu, C. Zhong, C. Ding, Q. Chi, A. Walz, P. Mombaerts, H. Matsunami, M. Luo, Detection of near-atmospheric concentrations of CO2 by an olfactory subsystem in the mouse. Science 317, 953-957 (2007). [Abstract] [Full Text]