Synaptic transmission in pair recordings from CA3 pyramidal cells in organotypic culture

We performed simultaneous whole cell recordings from pairs of monosynaptically coupled hippocampal CA3 pyramidal neurons in organotypic slices. Stimulation of an action potential in a presynaptic cell resulted in an AMPA-receptor-mediated excitatory postsynaptic current (EPSC) in the postsynaptic cell that averaged approximately 34 pA. The average size of EPSCs varied in amplitude over a 20-fold range across different pairs. Both paired-pulse facilitation and depression were observed in the synaptic current in response to two presynaptic action potentials delivered 50 ms apart, but the average usually was dominated by depression. In addition, the amplitude of the second EPSC depended on the amplitude of the first EPSC, indicating competition between successive events for a common resource that is not restored within the 50-ms interpulse interval. Variation in the synaptic strength among pairs could arise from a variety of sources. Our data from anatomic reconstruction, 1/CV2 analysis, paired-pulse analysis, and manipulations of calcium/magnesium ratio suggest that differences in quantal size and release probability do not appear to vary sufficiently to fully account for the observed differences in amplitude. Thus it seems most likely that the variability in EPSC amplitude between pairs arises primarily from differences in the number of functional synapses. Injections of the calcium chelator bis-(o-aminophenoxy)-N, N,N',N'-tetraacetic acid into the presynaptic neuron resulted in a rapid and nearly complete block of transmission, whereas injection of the slower-acting chelator EGTA resulted in a variable and partial block. In addition to demonstrating the feasibility of manipulating the intracellular presynaptic environment by injection into the presynaptic soma, these data, and the EGTA results in particular may suggest variability in the linkage between calcium entry sites an release sites in these synapses.