A mechanism intrinsic to the vesicle fusion machinery determines fast and slow transmitter release at a large CNS synapse.

Heterogeneity of release probability p between vesicles in the readily releasable pool (RRP) is expected to strongly influence the kinetics of depression at synapses, but the underlying mechanism(s) are not well understood. To test whether differences in the intrinsic Ca2+ sensitivity of vesicle fusion might cause heterogeneity of p, we made presynaptic Ca2+-uncaging measurements at the calyx of Held and analyzed the time course of transmitter release by EPSC deconvolution. Ca2+ uncaging, which produced spatially homogeneous elevations of [Ca2+]i, evoked a fast and a slow component of release over a wide range of [Ca2+]i, showing that mechanism(s) intrinsic to the vesicle fusion machinery cause fast and slow transmitter release. Surprisingly, the number of vesicles released in the fast component increased with Ca2+-uncaging stimuli of larger amplitudes, a finding that was most obvious below approximately 10 microM [Ca2+]i and that we call "submaximal release" of fast-releasable vesicles. During trains of action potential-like presynaptic depolarizations, submaximal release was also observed as an increase in the cumulative fast release at enhanced release probabilities. A model that assumes two separate subpools of RRP vesicles with different intrinsic Ca2+ sensitivities predicted the observed Ca2+ dependencies of fast and slow transmitter release but could not fully account for submaximal release. Thus, fast and slow transmitter release in response to prolonged [Ca2+]i elevations is caused by intrinsic differences between RRP vesicles, and an "a posteriori" reduction of the Ca2+ sensitivity of vesicle fusion after the onset of the stimulus might cause submaximal release of fast-releasable vesicles and contribute to short-term synaptic depression.