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

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 less than or equal to 5 ms of step onset. I-A was antagonized by external tetraethylammonium or 4-aminopyridine, and by intracellular caesium. The second population expressed a delayed rectifying potassium current (I-K) that reached peak amplitude greater than or equal to 10 ms after step onset and did not inactivate. I-K was antagonized by internal caesium and external tetraethylammonium. Bipolar cells expressing I-K also expressed a time-dependent h-current at membrane potentials <-50 mV. I-h was sensitive to external caesium and barium, and was also reduced by Na+-free Ringer, In both groups, a calcium current (I-Ca) and a calcium-dependent potassium current (I-K(Ca)) were identified. Depolarizing voltage steps >-50 mV activated I-Ca, which reached peak amplitude between -20 and -10 mV, I-Ca was eliminated in Ca2+-free Ringer and blocked by cadmium and cobalt, but not tetrodotoxin. In most cells, I-Ca 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 I-Ca. I-K(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.