Olfactory CO(2) chemoreceptors

Amphibians and reptiles possess CO(2)-sensitive olfactory receptors that cause a dose-dependent decrease in breathing when stimulated by CO(2) 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 CO(2)-sensitive olfactory receptors to transient changes in nasal CO(2). Histology and electrophysiology studies in frogs show that identification of sites of CA activity can serve as markers for locations of CO(2) chemosensitivity in the olfactory epithelium. There is also growing evidence that CO(2) 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 CO(2) 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 CO(2) in areas of the rat nasal cavity exhibiting the highest densities of CA activity; and (4) discuss the presumed role of the olfactory CO(2) receptors in the control of breathing and in abnormalities of breathing, such as sudden infant death syndrome (SIDS).