Figure 3.

PI evoked in vitro by the contralateral GABAergic synaptic inputs. A, Overlay of 10 traces of current-clamp recording from a DNLL neuron. Spikes were elicited via a continuous current injection of 200 pA for 1500 ms (top, black line). The fibers of the commissure of Probst were stimulated with a short train of three pulses at 500 Hz (top, gray line) to activate the GABAergic inhibitory input from the contralateral DNLL. Left, After the end of the fiber stimulation, the cell showed persistent spike suppression (63.3 ms; indicated by asterisk). Right, Bath application of the GABA_A receptor blocker SR95531 eliminated spike suppression. B, Simulation of sound-evoked excitatory inputs by repetitive 0.5 ms current injections (1–2 nA; at 100 Hz; top, black line), each eliciting one action potential (first indicated as “1”). When the commissure was stimulated by the same three-pulse protocol used above (top, gray line), action potentials to the two following current injections were suppressed (second failure indicated as “2”). The inset shows blowups of the first spike (1) and the second failure (2). C, Firing probability of six neurons before and after the recruitment of the GABAergic inhibition by the three-pulse protocol (gray line and symbols). The fiber stimulation (as in B) is indicated by the arrow. Black line, Control without fiber stimulation showing the high spiking fidelity of the neurons. Spikes were elicited every 10 ms. Error bars represent SEM. D, Voltage-clamp recordings from a DNLL neuron showing inhibitory postsynaptic currents in response to the same three-pulse fiber stimulation described above. Fiber stimulation elicited an IPSC of ∼2 nA whose decay could best be described by fitting a double exponential with time constants of 12.3 and 43.7 ms. Solid black line, Average of 10 traces; gray lines, single traces. The current could be blocked by SR95531 (dashed black line).