An antibody to synaptotagmin I facilitates synaptic transmission

The cytoplasmic domain of rat synaptotagmin I enhances synaptic transmission

Synaptotagmin, an integral membrane protein of synaptic vesicles, functions as a calcium sensor in the temporal control of neurotransmitter release. Although synaptotagmin facilitates lipid membrane fusion in biochemical experiments, overexpression of synaptotagmin inhibits neurotransmission. A facilitatory effect of synaptotagmin on synaptic transmission was never observed. To determine whether synaptotagmin may accelerate synaptic transmission in vivo, we injected the cytoplasmic domain of rat synaptotagmin I (CD-syt) into crayfish motor axons and tested the effect of CD-syt on synaptic response. We confirmed that CD-syt accelerates neuromuscular transmission. The injected preparation had larger synaptic potentials with shorter rise time. Experiments with varying calcium concentrations showed that CD-syt increased the maximum synaptic response of the neuromuscular synapses. Further tests on short-term plasticity of neuromuscular synapses revealed that CD-syt increases the release probability of the release-ready vesicles.

Determining Ca2+-sensor binding time and its variability in evoked neurotransmitter release

The speed and reliability of neuronal reactions are important factors for proper functioning of the nervous system. To understand how organisms use protein molecules to carry out very fast biological actions, we quantified single-molecule reaction time and its variability in synaptic transmission. From the synaptic delay of crayfish neuromuscular synapses the time for a few Ca(2+) ions to bind with their sensors in evoked neurotransmitter release was estimated. In standard crayfish saline at room temperature, the average Ca(2+) binding time was 0.12 ms for the first evoked quanta. At elevated extracellular Ca(2+) concentrations this binding time reached a limit due to saturation of Ca(2+) influx. Analysis of the synaptic delay variance at various Ca(2+) concentrations revealed that the variability of the Ca(2+)-sensor binding time is the major source of the temporal variability of synaptic transmission, and that the Ca(2+)-independent molecular reactions after Ca(2+) influx were less stochastic. The results provide insights into how organisms maximize reaction speed and reliability.