Figure 4

Different forms of plasticity at inhibitory synapse onto lamina II inhibitory neurons. Recordings were made in the presence of APV, CNQX and strychnine. A. Summation of evoked IPSCs (eIPSCs) caused by a train of 12 focal stimuli (dots at bottom) at 20 Hz. Stimulus artefacts are partially blanked. Note the occurrence of numerous sIPSCs at the end of the train indicative of asynchronous release. B. The normalized amplitude of the first eIPSC evoked by each train is plotted for the initial five consecutive repetitions of the trains (at 0.05 Hz; black). The amplitude of the first eIPSC in each train was normalized to the average of five IPSCs evoked by single stimuli (at 0.1 Hz) that preceded the train stimulation protocol. The values plotted are the means ± SEM from 20 neurons. The graph shows a significant (p < 0.05, Friedman test) increase in the eIPSC amplitude after the initial train (train number 1). Asterisks denote significant differences in subsequent values from the initial train (train number 1; Student-Newman-Keuls posthoc test; p < 0.05). For comparison, the normalized amplitudes of five consecutive IPSCs and the 13^th IPSC evoked by single stimuli are plotted (red). This indicates that the potentiation of the eIPSCs after the first train (of 12 stimuli) does not occur with accumulation of the same number of stimuli at low frequency. C. Left: Normalized amplitudes of the initial four eIPSCs within the first (black) and fifth (red) train. The values plotted are the means ± SEM from 20 neurons. Right: Ratios between the second eIPSC and the first eIPSC (eIPSC₂/eIPSC₁) from individual trains are plotted against the normalized amplitude of the first eIPSC. The data shows negative correlation between these values (linear fit: r = -0.53, p < 0.05), indicating depression of the second eIPSC when the first was potentiated and vice versa. D. Peristimulus time histogram showing the change in sIPSC frequency before and after each train (arrow; n = 15 trains from one cell).