Effects of Zn (10 and 100 μm) on the sEPSCs and the eEPSCs. A₁, Effects of Zn (10 and 100 μm) on eEPSCs (10 traces). CTL: a₁ −308 pA, a_{2 −}25.4 pA, τ₁ 10.6 ms, τ₂ 81.3 ms, τ_w 16.0 ms. Zn 10 μm: a₁ −293 pA, a₂ −25.9 pA, τ₁ 11.4 ms, τ₂ 78.4 ms, τ_w 16.8 ms. Zn 100 μm: a₁ −134 pA, a₂ −21.3 pA, τ₁ 12.6 ms, τ₂ 87.7 ms, τ_w 22.9 ms. A₂, Effects of Zn (100 μm) on the sEPSCs (inset) (same experiment as in A₁). The records illustrated correspond to sEPSCs recorded on a 150 ms time base and fit with a single exponential (control: 194 events, −12 pA, td 8.8 ms; Zn 10 μm: 356 events, −14.5 pA, td 9.1 ms; Zn 100 μm: 592 events −13 pA, td = 12.8 ms). The values of the kinetic parameters for a dual exponential fit were also measured on a 540 ms time base, but this reduced the number of events. B₁, Effects of Zn (100 μm) on eEPSCs. There was no significant change in the two time constants of the decay. The reduction of the amplitude was more important for a₁ than for a₂. B₂, Effects on eEPSCs of reducing the external Ca concentration from 2 to 1 mm. There was no significant change in the two time constants of the decay. The reduction of the amplitude was slightly higher for a₁ than for a₂, leading to a slight increase in the ratio a₂/a₁ (p = 0.016; n = 7). C, Ratio of the two amplitudes a₁ and a₂ in presence of Zn (100 μm) and in reduced Ca (1 mm). The ratio is increased by Zn from 0.14 ± 0.06 to 0.61 ± 0.41 (n = 7; p = 0.01; C₁) and unchanged (control 0.27 ± 0.12; reduced Ca 0.15 ± 0.03; n = 7; p = 0.07 bilateral paired t test) when the release is reduced presynaptically by lowering the Ca concentration (C₂).