Quantal transmission at purinergic synapses: stochastic interaction between ATP and its receptors

Monte Carlo methods are used to analyze the stochastic interaction between ATP, released in a packet at a bouton, and the underlying patch of purinoceptors. The time-course of the average quantal current recorded with an intracellular electrode in the peripheral and central nervous systems is reconstructed, given the geometry of the synapse and the known kinetics of ATP action. This leads to certain restrictions on the possible numbers of ATP molecules in a quantum and the density of purinoceptors at the synapses. The addition of an ectoenzyme into the synaptic cleft with the known kinetics of ATPase gives rise to a quantal current of appropriate time-course if the number of ATP molecules in a quantum is increased over that in the absence of the ATPase. The stochastic variability in the quantal current is less than 0.1 for a given size quantum.