Regulation of efficacy at central synapses.

The quantal nature of synaptic depression produced by high frequency stimulations has been analyzed at a central synapse for the first time. Simultaneous intracellular recordings were obtained from the Mauthner cell and adjacent identifiable inhibitory interneurons. The presynaptic cells were stimulated at frequencies from 2 to 33 Hz, and the corresponding release parameters were determined using a computational procedure described elsewhere (Korn, H., A. Triller, A. Mallet, and D. S. Faber (1981) Science 213: 898-901). As in our previous studies, these entities were correlated with histological features of the neurons following systematic horseradish peroxidase injections and reconstructions. Evidence was obtained that, in the range of physiological conditions used, the binomial parameter n (number of available quanta for release) remains constant; thus every synaptic bouton continues to function as an independent all-or-none releasing unit. The progressive reduction in amplitude of postsynaptic potentials can be attributed solely to a lower probability of release, as shown by the fall of the binomial parameter p. This evidence supports the concept that p is a critical variable for short-term modifications of synaptic efficacy and may provide insight for instances of synaptic plasticity underlying those behavioral changes which can be attributed to presynaptic loci. The present study also represents a necessary step toward linking mathematical variables of models for transmitter exocytosis with subcellular events.