ELECTRICAL RESONANCE AND CA2+ INFLUX IN THE SYNAPTIC TERMINAL OF DEPOLARIZING BIPOLAR CELLS FROM THE GOLDFISH RETINA

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 Ca2influx in the synaptic terminal. 2. The current-voltage relation was l inear up to about -44 mV. At this threshold, the injection of 1 pA of current triggered a maintained 'all-or-none' depolarization to a plate au of -34 mV, associated with a decrease in input resistance and a dam ped voltage oscillation with a frequency of 50-70 Hz and initial ampli tude of 4-10 mV. A second frequency component of 5-10 Hz was often obs erved. In a minority of cells the response to current injection was tr ansient, 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 -4 3 mV and free [Ca2+](i) in the synaptic terminal rose during a depolar izing response. Simultaneous measurements of the fluorescence associat ed with the membrane marker FM1-43 demonstrated that these Ca2+ signal s stimulated exocytosis. Regenerative depolarizations and associated r ises in [Ca2+], were blocked by inhibiting L-type Ca2+ channels with 3 0 mu M nifedipine. 5. Depolarization beyond -40 mV also elicited an ou twardly rectifying K+ current. Blocking this current by replacing exte rnal Ca2+ with Ba2+ caused the voltage reached during a depolarizing r esponse to increase to +10 mV. 6. The majority of the K+ current was b locked by 100 nM charybdotoxin, indicating that it was carried by larg e-conductance Ca2+-activated K+ channels. a transient voltage-gated K current remained, which began to activate at -40 mV. High-frequency v oltage oscillations were blocked by 100 nM charybdotoxin, but low-freq uency oscillations remained. 7. These results indicate that the voltag e response of depolarizing bipolar cells is shaped by L-type Ca2+ chan nels, Ca2+-activated K+ channels and voltage-dependent K+ channels. Th is combination of conductaaances regulates Ca2+ influx into the synapt ic terminal and confers an electrical resonance on the bipolar cell.