| A supralinear relationship between presynaptic calcium influx and neurotransmitter release (Ca2+-release relationship, CRR) often underlies inferences about the number and dynamics of Ca2+-binding proteins (sensors) that trigger exocytosis. A ∼4th order CRR holds for well-organized active zones (AZs) of the frog neuromuscular junction (NMJ) (Dodge and Rahamimoff, 1967). However when CRR was measured in regenerating NMJ's with altered AZ structure, a ∼3rd order relationship was measured. This suggests that the CRR may depend strongly on detailed spatial relationships between AZ components. To explore this possibility, Monte Carlo simulations of presynaptic Ca2+ dynamics and transmitter release were performed using MCell, a general Monte Carlo simulation program, and realistic models of frog NMJ active zones in both normal and various disorganized configurations. It was determined that sensors on synaptic vesicles respond to a localized wave of Ca2+ originating from nearby calcium channels opened by simulated nerve depolarization. The apparent order of the CRR depends heavily on the different magnitudes of the Ca2+ waves seen by individual vesicles, and therefore the distances between calcium channels and sensors, as well as the number of sensors on each vesicle. We conclude that AZ architecture has a major influence on apparent CRR, and that spatially realistic models are required to interpret experimental findings. |
