Fig. 2.

Relationship between the paired-pulse ratio and the amplitude of EPSP₁. The paired-pulse ratio (A, C) and the amplitude of EPSP₂ (B, D) are plotted versus the amplitude of EPSP₁ for nondepressed synapses (A, B) and depressed synapses (C, D). Both sets of data are for the first trial in which a synapse was tested with paired stimulation. Paired-pulse data on nondepressed synapses were obtained 10–15 min after a synapse first was identified. Synaptic depression was achieved first by stimulating an SN to fire 15 single action potentials at a 15 sec ITI; this protocol reduced synaptic strength to an average of 32.8 ± 1.6% of the initial amplitude (see Fig.4A for data on the time course of depression for these same synapses). The paired-pulse test was 15 sec after the fifteenth SN spike. For A and C, the data were fit with hyperbolic functions of the form a +b/x, where x is the amplitude of EPSP₁ (solid lines). InC, the dotted curve is the hyperbolic fit for nondepressed synapses from A; the dashed curve is the curve from A transformed by multiplying x by the average depression ratio (i.e., 0.328). In B and D, the data were fit with straight lines with a slope of 0.18 (with a correlation coefficient of 0.18) for the nondepressed synapses (B) and with a slope of 0.71 (with a correlation coefficient of 0.61) for the depressed synapses (D). Note that, whereas there is an inverse correlation between the paired-pulse ratio and the amplitude of EPSP₁, the amplitude of EPSP₁ is not negatively correlated with EPSP₂.