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 inne rvated membrane of the electric organ of Torpedo marmorata. Electrophy siological signals were compared to the predictions of a semi-quantita tive 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 t emporal patterns. 2. The amplitude distribution of EPCs evoked by acti vation of nerve terminals showed quantal steps. The time to peak of EP Cs was in most cases independent of amplitude, but in their decaying p hase a positive correlation was seen between half-decay time and ampli tude. Comparison with the model suggested that (i) a dynamic interacti on occurred at the end of the EPC between the fields of postsynaptic m embrane activated by individual quanta, and (ii) the sites of quantal release in the electric organ are separated from each other by 600-100 0 nm. 3. Spontaneous miniature electroplaque potentials (MEPPs) were r ecorded externally with the same type of loose patch electrode. The ma jority (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 exhi bited 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 small est to the largest events. Acetylcholinesterase was not blocked. 5. In addition to the fast MEPPs, spontaneous signals exhibiting a slow rat e of rise, or a slow rate of decay, or both were observed. They occurr ed at any time during the experiment, independently of the overall fre quency. Approximately 15% of the total number of events had a slow ris e but their decay phase was nevertheless rapid and could be ascribed t o the kinetics of receptors. These slow-rising MEPPs exhibited a varie ty of conformations: slow but smooth rise, sudden change of slope and sometimes several bumps or inflexions. Their average amplitude was sig nificantly smaller than that of the main population of fast MEPPs. 6. Composite MEPPs with multiple peaks as well as bursts of small MEPPs w ere 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 si mulate the entire variety of signals we had to assume that the MEPPs w ere generated by either synchronized or desynchronized emission of sma ll quantities of transmitter. The typical relationship observed betwee n amplitude and time course in the population of fast MEPPs suggested that the different amounts of transmitter composing a quantum were del ivered synchronously close to each other (either at the same spot or a t less than 200 nm apart); it is proposed that they acted on overlappi ng fields of receptors and that their responses summed up in a superad ditive manner. 8. Computer analysis of the slow-rising MEPPs was of pa rticular interest since their rapid decay phase indicated that the pos tsynaptic links (cholinesterase and receptors kinetics) were apparentl y not altered in this subpopulation. More probably their slow and ofte n irregular rate of rise arose from some desynchronization of the rele ase process. 9. It is concluded that at the nerve-electroplaque juncti on evoked transmitter release operates in the form of quanta containin g ca 10000 acetylcholine molecules; the quanta activate independent bu t closely adjacent postsynaptic fields. Each quantum is apparently com posed 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 o ccasionally occurs (slow-rising and composite MEPPs).