Opposite modulation of time course of quantal release in two parts of the same synapse by reactive oxygen species

Reactive oxygen species (ROS) are potent regulators of transmitter release in chemical synapses, but the mechanism of this action remains almost unknown. Presynaptic modulation can change either the release probability or the time course of quantal release, which was recently recognized as an efficient mechanism determining synaptic efficiency. The nonuniform structure and a big size of the frog neuromuscular junction make it a useful model to study the action of ROS in compartments different in release probability and in time course of transmitter release. The time course (or kinetics) of quantal release could be estimated by measuring the dispersion of the synaptic delays for evoked uniquantal endplate currents (EPCs) under low release probability. Using two-electrode recording technique, the action of ROS on kinetics and release probabilities were studied at the proximal and distal parts within the same neuromuscular junction. The stable ROS hydrogen peroxide (H2O2) increased the dispersion of synaptic delays of EPCs (i.e. desynchronized quantal release) within the distal part but decreased delay dispersion (synchronized quantal release) within the proximal part of the same synapse. Unlike the opposite modulation of kinetics, H2O2 reduced release probability in both distal and proximal parts. Since ATP is released from motor nerve terminals together with acetylcholine and can be involved in ROS signaling, we tested the presynaptic action of ATP. In the presence of the pro-oxidant Fe2+, extracellular ATP, similarly to H2O2, induced significant desynchronization of release in the distal regions. The antioxidant N-acetyl-cysteine attenuated the inhibitory action of ATP on release probability and abolished the action of H2O2 and ATP in the presence of Fe2+, on release kinetics. Our data suggest that ROS induced during muscle activity could change the time course of transmitter release along the motor nerve terminal to provide fine tuning of synaptic efficacy.