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). (C) 2001 Elsevier Science B.V. All rights reserved.