Fig. 5. Frequency-dependent short-term modification of EPSPs at different stages of postnatal development. a, In young (P14) cortex, stimulation of a presynaptic layer 5 pyramidal cell at 10, 20, and 40 Hz evoked EPSPs in a postsynaptic layer 5 cell that depressed at all stimulus frequencies. The resting membrane potential was −68 mV. The dots below the voltage traces mark the times of occurrences of presynaptic action potentials. b, In more mature (P28) cortex, stimulation of a presynaptic layer 5 pyramidal neuron evoked EPSPs in a postsynaptic layer 5 cell that facilitated at stimulus frequencies of 10, 20, and 40 Hz. c, Changes in short-term modification of EPSPs evoked in layer 5 neurons in P14 (n = 52), P18 (n = 9), P22 (n = 6), and P28 (n = 10) rats are shown. The filled circles are means (± SD). d, Comparison of means (± SD) of EPSP amplitude ratios for different pyramidal cell connections in layers 2/3 and 5 in P14–P15 (filled bars) and P28 (open bars) animals is shown. EPSP amplitude ratios were measured during 10 Hz stimulation of presynaptic cells. Significant (p < 0.05, two-tailed ttest) differences in the means of the amplitude ratios are marked withasterisks. The number of paired recordings for P14 and P28 animals was, respectively, 44 and 8 for L2/3 to L2/3 connections, 55 and 7 for L2/3 to L5 connections, and 52 and 9 for L5 to L5 connections.