SPACE AND TIME CHARACTERISTICS OF TRANSMITTER RELEASE AT THE NERVE-ELECTROPLAQUE JUNCTION OF TORPEDO

1. A loose patch electrode was used to stimulate axon terminals and to record evoked electroplaque currents (EPCs) in a limited area of innervated membrane of the electric organ of Torpedo marmorata. Electrophysiological signals were compared to the predictions of a semi-quantitative model of synaptic transmission which was designed to simulate the release of several packets of neurotransmitter molecules, at the same or at different sites of the synapse, synchronously or with various temporal patterns. 2. The amplitude distribution of EPCs evoked by activation of nerve terminals showed quantal steps. The time to peak of EPCs was in most cases independent of amplitude, but in their decaying phase a positive correlation was seen between half-decay time and amplitude. Comparison with the model suggested that (i) a dynamic interaction occurred at the end of the EPC between the fields of postsynaptic membrane activated by individual quanta, and (ii) the sites of quantal release in the electric organ are separated from each other by 600-1000 nm. 3. Spontaneous miniature electroplaque potentials (MEPPs) were recorded externally with the same type of loose patch electrode. The majority (75%) of external MEPPs displayed a homogeneous and rapid time course. This fast MEPP population had a mean time to peak of 0.43 ms, a half-decay time of 0.45 ms and a time constant of decay of 0.35 ms. 4. Despite homogeneous characteristics of time course, fast MEPPs exhibited a wide amplitude distribution with a main population which could be fitted by a Gaussian curve around 1 mV, and another population of small amplitude. Both the time-to-peak and the half-decay time of fast MEPPs showed a positive correlation with the amplitude from the smallest to the largest events. Acetylcholinesterase was not blocked. 5. In addition to the fast MEPPs, spontaneous signals exhibiting a slow rate of rise, or a slow rate of decay, or both were observed. They occurred at any time during the experiment, independently of the overall frequency. Approximately 15% of the total number of events had a slow rise but their decay phase was nevertheless rapid and could be ascribed to the kinetics of receptors. These slow-rising MEPPs exhibited a variety of conformations: slow but smooth rise, sudden change of slope and sometimes several bumps or inflexions. Their average amplitude was significantly smaller than that of the main population of fast MEPPs. 6. Composite MEPPs with multiple peaks as well as bursts of small MEPPs were often encountered, even during periods of low frequency. They were suggestive of a complete disorganization of quantal events. 7. Fast, slow and composite MEPPs were analysed using the computer model. To simulate the entire variety of signals we had to assume that the MEPPs were generated by either synchronized or desynchronized emission of small quantities of transmitter. The typical relationship observed between amplitude and time course in the population of fast MEPPs suggested that the different amounts of transmitter composing a quantum were delivered synchronously close to each other (either at the same spot or at less than 200 nm apart); it is proposed that they acted on overlapping fields of receptors and that their responses summed up in a superadditive manner. 8. Computer analysis of the slow-rising MEPPs was of particular interest since their rapid decay phase indicated that the postsynaptic links (cholinesterase and receptors kinetics) were apparently not altered in this subpopulation. More probably their slow and often irregular rate of rise arose from some desynchronization of the release process. 9. It is concluded that at the nerve-electroplaque junction evoked transmitter release operates in the form of quanta containing ca 10000 acetylcholine molecules; the quanta activate independent but closely adjacent postsynaptic fields. Each quantum is apparently composed of a preferential number of subunits emitted at the same point, or very close to each other. The subunits are delivered synchronously in the majority of events (fast MEPPs) but subunit desynchronization occasionally occurs (slow-rising and composite MEPPs).