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

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 microM 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 K+ 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(2+)-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.