Effects of mobile buffers on facilitation: Experimental and computational studies

Facilitation is an important form of short-term plasticity that occurs in m ost synapses. At crayfish neuromuscular junctions, basal transmission and f acilitation were significantly reduced after presynaptic introduction of "f ast" high-affinity calcium buffers, and the decay of facilitation was accel erated. The existence of residual calcium during facilitation was also demo nstrated. Computational modeling of three-dimensional buffered Ca2+ diffusi on and binding to secretory and facilitation targets suggest that the facil itation site is located away from a secretory trigger mediating exocytosis; otherwise, the facilitation site would be saturated by each action potenti al. Our simulations account for many characteristics of facilitation and ef fects of exogenous buffer, and suggest that facilitation is caused by resid ual calcium gaining access to a site distinct from the secretory trigger th rough restricted diffusion.