Effect of penicillin on the rate of block of GABA-responses by TBPS. Graph of the current amplitude vs sequential number of test GABA-applications (delivered 1/5 s). Upon application of penicillin (5 mM, open bar) response amplitude is immediately reduced to <50% of control. Application of TBPS (10 μM, closed bar) produces a rapid exponential block with an e-fold reduction in amplitude after 1.66 applications of GABA. Upon washout of TBPS, a slower, exponential recovery to control values is observed. After washout of penicillin and recovery of control amplitudes, TBPS is applied again inducing again a rapid decline in current amplitudes with an e-fold reduction in amplitude after 2.08 GABA-applications, i.e. it is slower than that observed in the presence of penicillin. Insets a and b show superimposed original current traces from the respective portions of the experiment: four traces recorded before TBPS-application are shown together with those during development of the block to the steady state. Data from the same experiment as in Figure 1.