We examined modulation of ionic currents by Zn2+ in acutely dissociated neurons from the rat's horizontal limb of the diagonal band of Broca using the whole-cell patch-clamp technique. Application of 50 mu M Zn2+ increased the peak amplitude of the transiently activated potassium current, I-A (at + 30 mV), from 2.20 +/- 0.08 to 2.57 +/- 0.11 nA (n = 27). This response was reversible and could be repeated in 0 Ca2+/1 mu M tetrodotoxin (n = 15). Zn2+ shifted the inactivation curve to the right, resulting in a shift in the half-inactivation voltage from - 76.4 +/- 2.2 to - 53.4 +/- 2.0 mV (n = 11), with no effect on the voltage dependence of activation gating (n = 15). There was no significant difference in the time to peak under control conditions (7.43 +/- 0.35 ms, n = 14) and in the presence of Zn2+ (8.20 +/- 0.57 ms, n = 14). Similarly, the time constant of decay of I-A (tau(d)) at + 30 mV showed no difference (control: 38.68 +/- 3.68 ms, n = 15; Zn2+: 38.48 +/- 2.85 ms, n = 15). I-A was blocked by 0.5-1 mM 4-aminopyridine. In contrast to its effects on I-A, Zn2+ reduced the amplitude of the delayed rectifier potassium current (I-K) The reduction of outward K+ currents was reproducible when cells were perfused with 1 mu M tetrodotoxin in a 0 Ca2+ external solution. The amplitude of the steady-state outward currents at + 30 mV under these conditions was reduced from 6.40 +/- 0.23 (control) to 5.76 +/- 0.18 nA in the presence of Zn2+ (n = 16). The amplitudes of peak sodium currents (I-Na) were not significantly influenced (n = 10), whereas barium currents (I-Ba) passing through calcium channels were potently modulated. Zn2+ reversibly reduced I-Ba at - 10 mV by similar to 85% from - 2.06 +/- 0.14 nA under control conditions to - 0.30 +/- 0.10 nA in the presence of Zn2+ (n = 14). Further analyses of Zn2+ effects on specific calcium channels reveals that it suppresses all types of high-voltage-activated Ca2+ currents. Under current-clamp conditions, application of Zn2+ resulted in an increase in excitability and loss of accommodation (n = 13), which appears to be mediated through its effects on Ca2+-dependent conductances. (C) 1999 IBRO. Published by Elsevier Science Ltd.
