Figure 5.

GABA_A-mediated fast synaptic transmission is not required for spike inhibition. (A) GABAergic IPSCs (top) evoked in FSI₁ at various holding potentials (from −100 to −20 mV, 10-mV steps; middle) in response to two consecutive AP elicited in FSI2 at an interval of 50 ms (bottom). In pairs connected by GABAergic synapses, mean peak amplitude, 10–90% rise time, and decay time constant of unitary IPSCs (recorded in voltage-clamp mode at V_clamp = −90 mV) were −70 ± 14 pA, 0.6 ± 0.08 ms, and 3.7 ± 0.5 ms, respectively, while the mean paired-pulse ratio was 0.85 ± 0.05. Bath application of 10 μM gabazine for 5–10 min completely blocked the IPSCs, unmasking small GJ-mediated currents. Inset, magnification of I_GJ (black arrowhead) followed by I_GABA (white arrowhead) recorded at −100 mV in response to an individual presynaptic AP (bottom trace) (B) Left, current-voltage plots of IPSC amplitudes (mean ± s.e.m.) normalized to average values recorded at −100 mV. Blue and black traces represent data obtained with whole-cell (n = 15) and perforated patch recordings (n = 4), respectively. Right, summary plot of average reversal potentials for Cl⁻ ions (E_Cl) obtained with perforated patch (−59.6 ± 3 mV, n = 4) and whole-cell recordings (−58.2 ± 2 mV; n = 15). The theoretical E_Cl calculated using the Nernst equation in our experimental conditions was −59 mV (see Methods). (C) Example of spike series in which inhibition is still evident during application of 10 μ M gabazine. (D) Left, voltage-clamp recordings of spikelets alone (i.e., without superimposed IPSCs) at various command potentials in response to three presynaptic AP (blue trace) in a pair connected by GJ, but not GABAergic synapses. Right, mutual inhibition of firing activity recorded in the same pair.