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

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO 2 levels as low as 0.13% ( mmHg), more than an order of magnitude lower than the typical arterial levels (∼40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO 2 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 CO 2 . An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO 2 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 CO 2 sensing in the mammalian carotid body. Further, the NEC response to CO 2 (magnitude of membrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by ∼50% by the CA-inhibitor acetazolamide. This suggests that the CO 2 detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO 2 and which does not require a change of extracellular pH. Because some cells that were responsive to increasing 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 CO 2 and O 2 .