Differential expression of voltage‐gated K+ and Ca2+ currents in bipolar cells in the zebrafish retinal slice

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Abstract
Whole‐cell voltage‐gated currents were recorded from bipolar cells in the zebrafish retinal slice. Two physiological populations of bipolar cells were identified. In the first, depolarizing voltage steps elicited a rapidly activating A‐current that reached peak amplitude ≤ 5 ms of step onset. IA was antagonized by external tetraethylammonium or 4‐aminopyridine, and by intracellular caesium. The second population expressed a delayed rectifying potassium current (IK) that reached peak amplitude ≥ 10 ms after step onset and did not inactivate. IK was antagonized by internal caesium and external tetraethylammonium. Bipolar cells expressing IK also expressed a time‐dependent h‐current at membrane potentials < – 50 mV. Ih was sensitive to external caesium and barium, and was also reduced by Na+‐free Ringer. In both groups, a calcium current (ICa) and a calcium‐dependent potassium current (IK(Ca)) were identified. Depolarizing voltage steps > – 50 mV activated ICa, which reached peak amplitude between – 20 and – 10 mV. ICa was eliminated in Ca+2‐free Ringer and blocked by cadmium and cobalt, but not tetrodotoxin. In most cells, ICa was transient, activating rapidly at – 50 mV. This current was antagonized by nickel. The remaining bipolar cells expressed a nifedipine‐sensitive sustained current that activated between – 40 and – 30 mV, with both slower kinetics and smaller amplitude than transient ICa. IK(Ca) was elicited by membrane depolarizations > – 20 mV. Bipolar cells in the zebrafish retinal slice preparation express an array of voltage‐gated currents which contribute to non‐linear I–V characteristics. The zebrafish retinal slice preparation is well‐suited to patch clamp analyses of membrane mechanisms and provides a suitable model for studying genetic defects in visual system development.