Figure 1.
Visual conditioning-induced bidirectional plasticity of GABAergic input. A, Left, Schematic diagram of a feedforward inhibitory circuit in the tectum. Local GABAergic interneurons (gray circle) receive excitatory drive from RGC inputs and make connections with other tectal neurons. Right, Example synaptic responses in a stage 43 tectal cell, each evoked by a 1.5 s whole-field light stimulus (represented by dark lines on top; On, change of screen color from black to white; Off, return to black) and recorded at Vh of −70 and 0 mV, respectively. Bottom trace, Response recorded at 0 mV was blocked after bath application of 100 μm picrotoxin. Calibration: 50 pA, 500 ms. B, Average integrated charges of whole-field light-evoked cISC and cESC at different developmental stages (stg.) (n = 12, 14, 17, 6 for groups 1–4). There is a significant decrease in cISC and a significant increase in cESC from stage 40–41 to 45–46 (p < 0.05). C, Left, In this example cell of a stage 40–41 tadpole, visual stimulus elicited a large cISC when recorded at 0 mV (bottom) but no significant glutamatergic response when recorded at −70 mV (top). Calibration: 60 pA, 500 ms. Right, Normalized integrated charge of cISC before and after visual conditioning (marked by the black arrow) in the same cell. Each data point was normalized to the control value (averaged during the 10 min period before the conditioning). Circles represent each sampled cISC. Squares represent average value for four consecutive cISCs. Traces on top are average responses (from 8 trials) before, during, and after conditioning, at times indicated by the open arrows. Calibration: 50 pA/20 mV, 250 ms. D, Summary of 13 similar experiments (filled symbols). Data represent mean ± SEM. Open symbols are for summary of nine control experiments in which cISCs were monitored continuously at Vh of 0 mV and at 0.04 Hz. E, Left, Light-evoked cESC and cISC (single trial) in an example cell at stage 45–46, recorded at −70 and 0 mV, respectively. Calibration: 40 pA, 200 ms. Right, Normalized integrated charge of cISC for the same cell before and after conditioning. Calibration: 40 pA/12 mV, 250 ms. F, Summary of 12 similar experiments. G, Top, Currents evoked by puffing GABA (indicated by the arrow) were completely blocked by picrotoxin (100 μm). Calibration: 40 pA, 500 ms. Bottom, GABA-evoked currents recorded at six different membrane potentials in an example cell. Each trace was average from three trials. Calibration: 30 pA, 200 ms. H, Current–voltage relationship for GABA-evoked currents before and 10–15 min after visual conditioning for the same cell. Data point represents average value of five to eight trials. Lines are best fits using linear regression. Black line is for after conditioning. The reversal potential for Cl− currents is taken as the x-intercept of the best-fit line. I, Measured Cl− equilibrium potentials in five experiments. The asterisk indicates the cell that exhibited a dominating cISC. Bars are for mean ± SEM.