FIG. 6.

Sustained depolarization triggered GABA release by activating Ca²⁺ current. A: DA cell was depolarized from –100 to –10 mV for 1 s (CsMeSO₃-based pipette solution containing 0.02 mM EGTA; E_Cl about −60 mV). Fast outward current events were riding on the slowly declining inward current. The fast events continued as inward currents after cessation of the depolarization, but they were very small because of the low Cl⁻ concentration in the pipette. Segments of the traces are magnified below (asterisks). SR95531 (20 μM) reversibly blocked the transient currents. Incompletely cancelled capacitive current and Na⁺ current at the onset of depolarization are truncated. B: in the same cell as in A, application of 1 mM exogenous GABA for 1 s activated an inward current at –100 mV and an outward current at −10 mV. C: DA cell was depolarized from –60 to 0 mV for 1 s (CsCl-based pipette solution containing 0.02 mM EGTA; E_Cl ∼ 0 mV). With equal concentration of Cl⁻ on either side of the membrane, the fast outward current events were absent during the depolarization, but the fast inward current events became much more prominent after the repolarization. A segment of the current trace after the depolarization is magnified in the box. Thus a sustained depolarization causes a long-lasting discharge of GABAergic currents. D: DA cell was depolarized from –80 to –20 mV for 1 s (CsMeSO₃-based pipette solution containing 0.02 mM EGTA; E_Cl about –60 mV). Traces from a sequence of experiments carried out in the same cell are shown in different colors. Black: in control conditions, transient events were superimposed on the slow inward current. Red: SR95531 (20 μM) blocked the transient events. Green: after SR95531 washout, Cd²⁺ (100 μM) blocked both the transient events and the slow inward current. Inset: after Cd²⁺ washout, both transient events and slow inward current were recovered. The slow current that follows the depolarization was blocked by Cd²⁺ but not by SR. It was probably caused by Na⁺-Ca²⁺ exchanger (Gleason et al. 1995). Again, because of the low intracellular Cl⁻, the events after repolarization were very small. Incompletely cancelled capacitive current and Na⁺ current at the onset of the depolarization are truncated. E: effects of Ca²⁺ channel blockers on Ca²⁺ current (black) and events of GABA release (white) activated by depolarization to –20 mV (Cd, cadmium; Co, cobalt substitution for Ca²⁺; vera, verapamil; nimo, nimodipine; SNX, SNX-482; aga, ω-agatoxin; cono, ω-conotoxin). Multiple types of Ca²⁺ channels contributed to the activation of the Ca²⁺ current elicited by the depolarization. Both L- and R-type channels seem to participate in the release of GABA.