Slices were transferred to a recording chamber at 21–24°C. During recordings, the standard perfusion solution consisted of the bicarbonate-buffered solution (see above) with 1 μM strychnine and 25 μM bicuculline to block inhibitory synaptic transmission. 1 mM kynurenic acid and 0.1 mM cyclothiazide (Tocris Bioscience/R&D Systems, Minneapolis, MN) were also added to block AMPA receptor saturation and desensitization, respectively. Slices were superfused at 1–3 ml/min with this external solution. Whole-cell postsynaptic patch-clamp recordings were made from visually identified cells in the MNTB region using glass pipettes of 2–3 MΩ resistance, filled with an internal recording solution consisting of the following (in mM): 110 CsCl, 35 CsF, 10 EGTA, 10 HEPES, 2 QX-314, pH: 7.2, 315–320 mOsm. Series resistance (Rs) was compensated by up to 60% and the membrane potential was held at −60 mV. Excitatory postsynaptic potentials (EPSCs) were evoked by stimulating presynaptic axons with a bipolar stimulating electrode placed midway between the medial border of the MNTB and the midline of the brainstem. A Multiclamp 700A (Axon Instruments/Molecular Devices, Union City, CA) amplifier was used. Recordings were digitized at 20 KHz with an ITC-18 A/D converter (Instrutech, Port Washington, NY) using custom procedures (written by M.A. Xu-Friedman) in IgorPro (Wavemetrics, Lake Oswego, OR) and filtered at 8 kHz. Access resistance and leak current were monitored and experiments were rejected if either parameter changed significantly. Recordings were performed at room temperature (25°C) in Figures 1-6 and 8B and at 35°C in Figure 7 and 8C.