Axonal GABA_ARs activation in single presynaptic varicosities has a marked effect on MLI excitability. (A; left) Representative traces of laser-evoked ASPs recorded in current-clamp mode with an intracellular solution containing [Cl⁻]_i = 15 mM. In this cell, photolysis of caged Ca²⁺ produced subthreshold depolarizing responses (same cell as in Fig. 6 A), with a P_spike of 0. (Right) Laser-evoked ASPs recorded in the same condition as shown in the left panel. In this cell, the ASPs induced AP firing with a probability P_spike of 0.625. (B) Summary plot of the fraction of release sites that produced active responses with 15 and 25 mM [Cl⁻]_i. The percentage of release sites that produced active responses in each experimental condition is shown in the bars (n = 17 sites with [Cl⁻]_i = 15 mM and 13 sites with [Cl⁻]_i = 25 mM). (C) Example of an MLI where GABA was photolyzed from DPNI-GABA (1 ms, 2-mW pulses) in four different axonal locations. Traces are averages from four to five sweeps, with amplitudes 5.5, 4.0, 1.7, and 0.6 mV; Vm values −70.8, −70.7, −67, and −72 mV (average values during 100 ms before the laser pulse); and distances to the soma 28.9, 41, 48, and 90 µm, respectively. Gray dotted line indicates timing of the laser pulse. (D) Somatic whole-cell recordings in current-clamp mode of the responses to current injection without (left) and with (right) prepulses of caged-GABA photolysis in the axon (50-ms interval between photolysis pulse and current injection). It can be observed that P_spike increases dramatically with the GABA prepulse. (E) P_spike in control (Ctrl) and with caged-GABA prepulses (GABA) for different time intervals between the laser pulse and the current injection: 50, 100, 200, and 300 ms; **, P < 0.01; ***, P < 0.001. (F) The increase of P_spike produced by the photolysis of caged GABA was plotted as a function of the interval between the laser and the current-injection pulses. The excitability increase has a very similar time course when compared with the decay of the GABA-induced depolarizations. Gray circles are the values from individual cells; black symbols and error bars represent mean ± SEM for each time interval. The cells were held near −70 mV.