New insights into short-term synaptic facilitation at the frog neuromuscular

25 26 Short-term synaptic facilitation occurs during high frequency stimulation, is known to be dependent on 27 presynaptic calcium ions, and persists for tens of milliseconds after a presynaptic action potential. We 28 have used the frog neuromuscular junction as a model synapse for both experimental and computer 29 simulation studies aimed at testing various mechanistic hypotheses proposed to underlie short-term 30 synaptic facilitation. Building off our recently reported excess-calcium-binding-site model of synaptic 31 vesicle release at the frog neuromuscular junction (Dittrich et al., 2013), we have investigated several 32 mechanisms of short-term facilitation at the frog neuromuscular junction. Our studies place constraints 33 on previously proposed facilitation mechanisms, and conclude that the presence of a second class of 34 calcium sensor proteins distinct from synaptotagmin, can explain known properties of facilitation 35 observed at the frog neuromuscular junction. We were further able to identify a novel facilitation 36 mechanism, which relied on the persistent binding of calcium bound synaptotagmin molecules to lipids 37 of the presynaptic membrane. In a real physiological context, both mechanisms identified in our study 38 (and perhaps others) may act simultaneously to cause the experimentally observed facilitation. In 39 summary, using a combination of computer simulations and physiological recordings, we have 40 developed a stochastic computer model of synaptic transmission at the frog neuromuscular junction, 41 which sheds light on the facilitation mechanisms in this model synapse. 42 43 44