Brevity of the Ca Microdomain and Active Zone Geometry Prevent Ca -Sensor Saturation for Neurotransmitter Release

Shahrezaei, Vahid and Kerry R. Delaney. Brevity of the Ca microdomain and active zone geometry prevent Ca -sensor saturation for neurotransmitter release. J Neurophysiol 94: 1912–1919, 2005. First published May 11, 2005; 10.1152/jn.00256.2005. The brief time course of the calcium (Ca ) channel opening combined with the molecular-level colocalization of Ca channels and synaptic vesicles in presynaptic terminals predict sub-millisecond calcium concentration ([Ca ]) transients of 100 M in the immediate vicinity of the vesicle. This [Ca ] is much higher than some of the recent estimates for the equilibrium dissociation constant of the Ca sensor(s) that control neurotransmitter release, suggesting release should be close to saturation, yet it is well known that release is highly sensitive to changes in Ca influx. We show that due to the brevity of the Ca influx the binding kinetics of the Ca sensor rather than its equilibrium affinity determine receptor occupancy. For physiologically relevant Ca currents and forward Ca binding rates, the effective affinity of the Ca sensor can be several-fold lower than the equilibrium affinity. Using simple models, we show redundant copies of the binding sites increase effective affinity of the Ca sensor for release. Our results predict that different levels of expression of Ca binding sites could account for apparent differences in Ca sensor affinities between synapses. Using Monte Carlo simulations of Ca dynamics with nanometer resolution, we demonstrate that these kinetic constraints combined with vesicles acting as diffusion barriers can prevent saturation of the Ca -sensor(s) for neurotransmitter release. We further show the random positioning of the Ca -sensor molecules around the vesicle can result in the emergence of two distinct populations of the vesicles with low and high release probability. These considerations allow experimental evidence for the Ca channel-vesicle colocalization to be reconciled with a high equilibrium affinity for the Ca sensor of the release machinery.