The steep dependence of exocytosis on Ca(2+) entry at nerve terminals implies that voltage control of both Ca(2+) channel opening and the driving force for Ca(2+) entry are powerful levers in sculpting synaptic efficacy. Using fast, genetically encoded voltage indicators in dissociated primary neurons, we show that at small nerve terminals K(+) channels constrain the peak voltage of the presynaptic action potential (APSYN) to values much lower than those at cell somas. This key APSYN property additionally shows adaptive plasticity: manipulations that increase presynaptic Ca(2+) channel abundance and release probability result in a commensurate lowering of the APSYN peak and narrowing of the waveform, while manipulations that decrease presynaptic Ca(2+) channel abundance do the opposite. This modulation is eliminated upon blockade of Kv3.1 and Kv1 channels. Our studies thus reveal that adaptive plasticity in the APSYN waveform serves as an important regulator of synaptic function.
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