Olfactory CO2 chemoreceptors

Amphibians and reptiles possess CO₂-sensitive olfactory receptors that cause a dose-dependent decrease in breathing when stimulated by CO₂ concentrations ranging from 0.5 to 8%. In amphibians, it has been shown that inhibition of the enzyme, carbonic anhydrase (CA), attenuates the response of CO₂-sensitive olfactory receptors to transient changes in nasal CO₂. Histology and electrophysiology studies in frogs show that identification of sites of CA activity can serve as markers for locations of CO₂ chemosensitivity in the olfactory epithelium. There is also growing evidence that CO₂ receptors may be present in the olfactory epithelium of mammals. The objectives of this review are to, (1) summarize the current state of knowledge of olfactory CO₂ receptors in amphibians, reptiles, and mammals; (2) present results from an experiment designed to determine the distribution and density of CA activity within the rat nasal cavity; (3) show results from an experiment that recorded the olfactory receptor response to CO₂ in areas of the rat nasal cavity exhibiting the highest densities of CA activity; and (4) discuss the presumed role of the olfactory CO₂ receptors in the control of breathing and in abnormalities of breathing, such as sudden infant death syndrome (SIDS).