Zebrafish (Danio rerio) gill neuroepithelial cells are sensitive chemoreceptors for environmental CO2

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO2 levels as low as 0.13% (P-CO2 = 1.0 mmHg), more than an order of magnitude lower than the typical arterial P-CO2 levels (similar to 40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO2 levels is not clear. Here, we show that the neuroepithelial cells (NECs) of the zebrafish gill, known to sense O-2 levels, also respond to low levels of CO2. An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO2 levels, a background K+ channel was inhibited, resulting in a partial pressure-dependent depolarization of the NEC. To elucidate the signalling pathway underlying K+ channel inhibition, we used immunocytochemistry to show that these NECs express carbonic anhydrase (CA), an enzyme involved in CO2 sensing in the mammalian carotid body. Further, the NEC response to CO2 (magnitude ofmembrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by similar to 50% by the CA-inhibitor acetazolamide. This suggests that the CO2 detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO2 and which does not require a change of extracellular pH. Because some cells that were responsive to increasing P-CO2 also responded to hypoxia with membrane depolarization, the present results demonstrate that a subset of the NECs in the zebrafish gill are bimodal sensors of CO2 and O-2.