P/Q KO mice have reduced evoked postsynaptic current (EPSC) and greatly diminished paired-pulse facilitation. EPSCs were evoked in MNTB neurons by stimulation with a bipolar electrode placed at the midline of the brainstem slice. A, Stimuli of 0.1 msec duration and 3-10 V amplitude were applied every 10-30 sec. Currents were recorded under whole-cell voltage-clamp conditions at a holding potential of -70 mV. The EPSC mean amplitude is 5.8 ± 0.6 nA (n = 7) for WT and 3.9 ± 0.4 nA (n = 7) for the KO, giving a 33% reduction in the synaptic current from KO mice with respect to WT. Differences in the 10-90% rise time and decaying time constants are not statistically significant (see Results). B, A pair of stimuli (as in A) was applied with a short interval (10 msec). Left, In low external Ca²⁺ concentration (0.6 mm) and high external Mg²⁺ concentration (2 mm), the WT EPSC evoked by the second stimulus is facilitated with respect to the first EPSC. In contrast, no facilitation is observed in KO cells. Right, Mean magnitude of facilitation is 25 ± 5% (n = 5) for WT and -6.2 ± 6.8% (n = 6) for the KO. C, When the release probability is further attenuated by increasing the external Mg²⁺ concentration to 3 mm, a small facilitation can be measured in KO mice. Left, Sample recordings of EPSCs from WT and KO mice. Right, Mean magnitude of facilitation between the second and first pulse is 26.9 ± 4% in WT mice (n = 3) and 7.8 ± 4.3% in KO (n = 3). Between the third and first pulse, the magnitude of facilitation is 55 ± 7% for WT and 23 ± 9% for KO.