Topical Inhibition of Nasal Carbonic Anhydrase Affects the CO2 Detection Threshold in Rats

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Previous studies indicate that Long-Evans rats can be operantly trained to discriminate inspired CO2 concentrations as low as 0.5%. This ability has been proposed to be due to the presence of CO2-sensitive olfactory receptors that contain the enzyme carbonic anhydrase (CA). The objectives of the present study were as follows: 1) to determine whether Zucker rats could be operantly conditioned to discriminate low concentrations of CO2 from control air and 2) to determine the rats’ CO2 detection thresholds before and after nasal perfusion of mammalian Ringers or methazolamide, a CA inhibitor. Rats were operantly trained to discriminate between 25% CO2 and control air (0% CO2) and were then subjected to various CO2 concentrations (0.5–12.5%) to determine their CO2 detection thresholds. The average (±standard error of mean) baseline CO2 detection threshold of 7 Zucker rats was 0.48 ± 0.07% CO2, whereas the average CO2 detection thresholds after nasal perfusion of either mammalian Ringers or 10−2 M methazolamide were 1.41 ± 0.30% and 5.92 ± 0.70% CO2, respectively. The average CO2 detection threshold after methazolamide was significantly greater (P < 0.0001) than the baseline detection threshold. These findings demonstrate that like Long–Evans rats, Zucker rats can be trained to discriminate low concentrations of CO2 and that inhibition of nasal CA reduces the ability of the rats to detect low concentrations (3.5% and below) but not higher concentrations of CO2 (12.5%). These results add to the growing evidence that olfactory neurons exhibiting CA activity are CO2 chemoreceptors sensitive to physiological concentrations of CO2.

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