Electrical resonance and Ca2+ influx in the synaptic terminal of depolarizing bipolar cells from the Goldfish retina

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Abstract
1 Whole‐cell recordings and fura‐2 measurements of cytoplasmic [Ca2+] were made in depolarizing bipolar cells isolated from the retinae of goldfish. The aim was to study the voltage signal that regulates Ca2+ influx in the synaptic terminal. 2 The current‐voltage relation was linear up to about −44 mV. At this threshold, the injection of 1 pA of current triggered a maintained ‘all‐or‐none’ depolarization to a plateau of −34 mV, associated with a decrease in input resistance and a damped voltage oscillation with a frequency of 50–70 Hz and initial amplitude of 4–10 mV. A second frequency component of 5–10 Hz was often observed. In a minority of cells the response to current injection was transient, recovering with an undershoot. 3 Unstimulated bipolar cells generated similar voltage signals, driven by current entering the cell through a non‐specific cation conductance that continuously varied in amplitude. 4 The threshold for activation of the Ca2+ current was −43 mV and free [Ca2+]i in the synaptic terminal rose during a depolarizing response. Simultaneous measurements of the fluorescence associated with the membrane marker FM1–43 demonstrated that these Ca2+ signals stimulated exocytosis. Regenerative depolarizations and associated rises in [Ca2+]i were blocked by inhibiting l‐type Ca2+ channels with 30 μm nifedipine. 5 Depolarization beyond −40 mV also elicited an outwardly rectifying K+ current. Blocking this current by replacing external Ca2+ with Ba2+ caused the voltage reached during a depolarizing response to increase to +10 mV. 6 The majority of the K+ current was blocked by 100 nm charybdotoxin, indicating that it was carried by large‐conductance Ca2+‐activated K+ channels. A transient voltage‐gated K2+current remained, which began to activate at −40 mV. High‐frequency voltage oscillations were blocked by 100 nm charybdotoxin, but low‐frequency oscillations remained. 7 These results indicate that the voltage response of depolarizing bipolar cells is shaped by l‐type Ca2+ channels, Ca+‐activated K+ channels and voltage‐dependent K+ channels. This combination of conductances regulates Ca2+ influx into the synaptic terminal and confers an electrical resonance on the bipolar cell.