RT Journal Article
SR Electronic
T1 Presynaptic Capacitance Measurements and Ca2+ Uncaging Reveal Submillisecond Exocytosis Kinetics and Characterize the Ca2+ Sensitivity of Vesicle Pool Depletion at a Fast CNS Synapse
JF The Journal of Neuroscience
JO J. Neurosci.
FD Society for Neuroscience
SP 7059
OP 7068
DO 10.1523/JNEUROSCI.23-18-07059.2003
VO 23
IS 18
A1 Wölfel, Markus
A1 Schneggenburger, Ralf
YR 2003
UL http://www.jneurosci.org/content/23/18/7059.abstract
AB The intracellular Ca2+ sensitivity of synaptic vesicle fusion is an important determinant of transmitter release probability, but it is unknown for most CNS synapses. We combined whole-cell membrane capacitance measurements and Ca2+ uncaging at the large calyx of Held nerve terminals to determine the Ca2+ sensitivity of synaptic vesicle fusion at a glutamatergic CNS synapse, independent of recording EPSCs. Capacitance increases measured 30-50 msec after elevating the intracellular Ca2+ concentration ([Ca2+]i) by Ca2+ uncaging were half-maximal at ∼5 μm [Ca2+]i. At 10 μm [Ca2+]i, capacitance increases reached maximal values (256 ± 125 fF; mean ± SD), indicating the depletion of an average pool of ∼4000 readily releasable vesicles. Vesicle pool depletion was confirmed in cross-depletion experiments, in which capacitance responses were measured after Ca2+ uncaging, or after combined stimuli of prolonged presynaptic depolarizations and Ca2+ uncaging. To analyze the Ca2+-dependent rates of vesicle pool depletion, the capacitance rise after Ca2+ uncaging was fitted with single- or double-exponential functions. The fast time constants of double-exponential fits, and the time constants of single-exponential fits were 2-3 msec at 10-15 μm [Ca2+]i and reached submillisecond values at 30 μm [Ca2+]i. These results suggest that three to five readily releasable vesicles can fuse within <1 msec at each active zone of a calyx of Held, given that [Ca2+]i rises sufficiently high. Submillisecond kinetics of exocytosis are reached at significantly lower [Ca2+]i than at ribbon-type sensory synapses previously investigated by capacitance measurements.