1. Coincidence-detection of excitatory synaptic potentials has long been considered to be the mechanism by which medial superior olivary (MSO) neurons compute interaural time differences. Here we demonstrate the contribution of synaptic inhibition in this circuit using a gerbil brain slice preparation. 2. Nearly all cells exhibited excitatory postsynaptic potentials (EPSPs) and action potentials (APs) after stimulation of either the ipsilateral or contralateral afferent pathway. In 44% of cells, the latency of APs depended on stimulus amplitude, exhibiting shifts of 0.25-2 ms. 3. Nearly all neurons (89%) exhibited stimulus-evoked synaptic inhibition. The inhibitory effects were enhanced at greater stimulus amplitudes and were usually able to block synaptically evoked APs. In addition, APs and EPSPs were reversibly blocked by delivering the inhibitory transmitter glycine in almost all tested cells (91%). 4. In the presence of the glycine antagonist strychnine, the effects of synaptic inhibition were suppressed. 5. The stimulus level-dependent inhibitory potentials influenced the probability that an MSO neuron would fire an AP, as well as the precise timing. Therefore, the present results have implications for the processing of interaural time differences by the MSO and at higher auditory centers.
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