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Published online: 5 September 2007 | doi:10.1038/nchina.2007.181

Olfactory system: Mice on the scent

Wei Zeng

Introduction

© (2007) istockphoto.com/Dan Brandenburg

Humans cannot smell CO₂, however, some animals can detect it at relatively high levels. A study led by Minmin Luo at the National Institute of Biological Sciences in Beijing has shown that mice can detect near-atmospheric levels of CO₂ through specialized neurons in the nose¹.

These specialized neurons, called guanylyl cyclase D (GC-D⁺) neurons, are known to express the enzyme carbonic anhydrase type II (CAII). When GC-D⁺ neurons are exposed to CO₂, CAII converts CO₂ into intracellular ions, HCO^-and H⁺, which in turn trigger and enhance calcium uptake by neurons in the olfactory bulb — a part of the brain responsible for the perception of odours. By using a fluorescent calcium-based dye, the researchers could see the CO₂ response and the whereabouts of the bulbar neurons. They found that almost all of the excited neurons were near the 'necklace glomeruli' region of the olfactory bulb.

The researchers also trained water-deprived mice to lick a port for water delivery during pulses of air with a 0.5% CO₂ concentration and not to lick during control pulses with the same flow rate and oxygen concentrations. By studying the behavioural change, the researchers showed that mice can detect CO₂ at levels as low as 0.066% (about twice that of atmospheric level).

Some studies have predicted that the global CO₂ level will exceed 0.066% within a century. It would be interesting to see whether this change has any impact on mice behaviour.

The authors of this work are from:
National Institute of Biological Sciences, Beijing, China; Institute of Biophysics, Chinese Academy of Sciences, Beijing, China; Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina, USA; The Rockefeller University, New York City, New York, USA.

Reference

1. Hu, J. et al. Detection of near-atmospheric concentrations of CO₂ by an olfactory subsystem in the mouse. Science 317, 953–957 (2007).  | Article | PubMed | ChemPort |