Intracellular calcium dependence of transmitter release rates at a fast central synapse

Abstract

Calcium-triggered fusion of synaptic vesicles and neurotransmitter release are fundamental signalling steps in the central nervous system. It is generally assumed that fast transmitter release is triggered by elevations in intracellular calcium concentration ([Ca2+]i) to at least 100 microM near the sites of vesicle fusion. For synapses in the central nervous system, however, there are no experimental estimates of this local [Ca2+]i signal. Here we show, by using calcium ion uncaging in the large synaptic terminals of the calyx of Held, that step-like elevations to only 10 microM [Ca2+]i induce fast transmitter release, which depletes around 80% of a pool of available vesicles in less than 3 ms. Kinetic analysis of transmitter release rates after [Ca2+]i steps revealed the rate constants for calcium binding and vesicle fusion. These show that transient (around 0.5 ms) local elevations of [Ca2+]i to peak values as low as 25 microM can account for transmitter release during single presynaptic action potentials. The calcium sensors for vesicle fusion are far from saturation at normal release probability. This non-saturation, and the high intracellular calcium cooperativity in triggering vesicle fusion, make fast synaptic transmission very sensitive to modulation by changes in local [Ca2+]i.

Time course and calcium dependence of transmitter release at a single ribbon synapse

“…The delay in onset and reduction in the rate of release may imply that a lower concentration of Ca 2ϩ reaches the Ca 2ϩ sensor in 5 mM EGTA, thereby delaying the readily releasable pool. This effect has been demonstrated at the calyx of Held, using a defined range of Ca 2ϩ concentrations presynaptically (36,37). The most straightforward interpretation of our results is that the Ca 2ϩ sensor (vesicle) is located at some distance from the Ca 2ϩ channel.…”

“…Thus, we assume that the Ca 2ϩ sensor is saturated and the postsynaptic response grows in linear proportion to the presynaptic Ca 2ϩ current because additional Ca 2ϩ channel openings bring with them their own ''unit'' of vesicular release. On the contrary, in a microdomain scenario, multiple channels sum to produce a Ca 2ϩ rise that reaches more distant vesicles, resulting in a super linear Ca 2ϩ dependence of release as found for the calyx of Held (35,36), where Ϸ60 channels open for each vesicle that is released (37,38). In this type of synapse, release is affected by Ca 2ϩ buffering with EGTA.…”

Miro1‐dependent mitochondrial positioning drives the rescaling of presynaptic Ca 2+ signals during homeostatic plasticity

“…Presynaptic Ca 2+ signals trigger vesicular release and their amplitude is known to influence synaptic transmission 4, 5. Previous pharmacological studies suggest that mitochondria can efficiently buffer Ca 2+ in presynaptic terminals 6, 7.…”

The timing of phasic transmitter release is Ca 2+ -dependent and lacks a direct influence of presynaptic membrane potential

“…The resulting Ca 2ϩ waveform had a 10-90% rise time of 0.95 ms, and did not decay Ͼ5% in the first 10 ms after its peak. This [Ca 2ϩ ] i waveform was used to drive a kinetic model (11,14) that assumes that 5 Ca 2ϩ ions bind cooperatively to the Ca 2ϩ sensor for vesicle fusion before an irreversible fusion step. By fitting the control data ( Fig.…”