Nature Neurosci. 1, 29– 35 (1998).

In preparing Fig. 2a, the same presynaptic traces were inadvertently used for all three panels. The correct version, which is virtually identical to our published Fig. 2a, is shown here. The authors regret the error.

Also, because of an editorial error, the x-axes in Figs 2b, 2c, 2d, 3a, 3b, 3e and 5c were mislabeled as 'ms.' The labels should read 'min.'