Venkatesh N. Murthy, Terrence J. Sejnowski and Charles F. Stevens.
The Salk Institute
We have used endocytotic uptake of the styryl dye FM1-43 at synaptic terminals (Betz & Bewick, 1992) to study properties of individual synapses formed by axons of single hippocampal neurons in tissue culture. The distribution of values for probability of evoked transmitter release p estimated by dye uptake is continuous, with a preponderance of low p synapses and a broad spread of probabilities. We have validated this method by demonstrating that the optically estimated distribution of p at autapses in single-neuron microislands predicts, with no free parameters, the rate of blocking of NMDA responses by the non-competitive antagonist MK-801 at the same synapses. Different synapses made by a single axon exhibited varying amounts of paired- pulse modulation; synapses with low p tended to be facilitated more than those with high p. The increment in release probability produced by increasing external calcium ion concentration was also dependent on a synapse's initial p value. The size of the recycling pool of vesicles was strongly correlated with p as well, suggesting that synapses with higher release probabilities had more vesicles. Finally, p values of neighboring synapses were correlated, indicating local interactions in the dendrite or axon, or both.