Quantal currents at single-site central synapses

Abstract

The mode of operation of synaptic transmission has been primarily worked out at the vertebrate neuromuscular junction, thus providing a framework for the interpretation of studies at central synapses. However, differences have been found between the two systems, and a coherent model is still lacking for central synapses. Research in this area revolves around several questions. (1) Is the variability of quantal amplitudes determined pre- or postsynaptically? (2) What is the occupancy of postsynaptic receptors following the release of a synaptic vesicle? And (3) does multivesicular release occur at single release sites following one presynaptic action potential? To answer these questions, it is essential to investigate synaptic processes at the level of single release sites. This is technically difficult because of the complex morphology and small dimensions of central synapses. Nevertheless significant advances have been made in the past few years.

Figure 1. Loose-patch recording of single quanta at individual hippocampal synapses

A, individual boutons are visualised along dendrites of cultured hippocampal neurones with the exocytosis-endocytosis marker FM1-43. A blunt pipette is placed on a well-isolated bouton for recording. B, during loose-patch recording, the pipette exclusively collects signals originating in the enclosed bouton. C, simultaneous local loose-patch recording (lp, upper trace) and somatic whole-cell recording (WC, lower trace). Miniature EPSCs originating in the enclosed bouton give rise to simultaneous, proportionally sized events in the lp and WC traces. D, amplitude histograms for simultaneously recorded signals from the lp and WC traces. Both histograms have Gaussian distributions and have similar CVs. Inset, cumulative histograms for the same data, showing in addition the population histogram recorded in the whole-cell trace. Note that the population histogram is much more broadly distributed than the histogram corresponding to the single bouton events. From Forti et al. 1997. Reprinted by permission from Nature (388,874-878) copyright 1997 Macmillan Magazines Ltd.

Figure 2. Measuring receptor occupancy from analysis of closely interspaced miniature events

A, principle of the measurement. Bursts of mIPSCs are elicited by a liminal application of α-LTX. Events that occur during the decay of a preceding mIPSC do not sum linearly with it; rather, they appear to have lower amplitudes than isolated events. For short values of the interval δt, the foot-to-peak amplitude A′₂ of the second event is smaller than that of the first event, while the baseline-to-peak amplitudes (A₂ and A₁) are similar. B, plot of A₂ as a function of δt, showing no significant increase at short time intervals. C, a plot of A′₂ as a function of δt can be approached with the sum of two exponentials. The slow exponential has a small amplitude and reflects cumulated desensitization of receptors during high frequency periods; the fast exponential component has a larger amplitude and reflects competition of successive release events for a common pool of postsynaptic receptors. The peak occupancy of postsynaptic receptors, ω_o, can be calculated from the extrapolated amplitudes for each component, A and A′, by using the equation in the figure. Reproduced with permission from Auger & Marty (1997), copyright Cell Press.

Figure 3. Multivesicular release at a single site synapse

A, recording of evoked IPSCs in a cerebellar interneurone following electrical stimulations in the molecular layer. Responses to 50 consecutive stimulations. Note the high proportion of failures (66 %), and the total lack of interference of the results with spontaneous synaptic currents. B, amplitude histograms for the responses in A. This histogram can be fitted with a single Gaussian distribution (CV = 20 %). The histogram of the recording noise is also shown. C, when displayed on a fast time scale, about 5 % of the successful responses in A can be seen to include two events in close succession. D, occurrence histograms of event pairs (as illustrated in C) as a function of interevent interval. The frequency of pairs falls off abruptly over a period of 3 ms. The dashed line illustrates the frequency of pairs expected on the basis of random summation of evoked events with the background activity due to unstimulated synapses. E, A₂/A₁ ratio as a function of interevent interval, from the same data. Reproduced with permission from Auger et al. (1998).