Fig. 3. (A ) Representative recordings for use-dependent block of tonicaine and QX-314. Tracings were recorded at the following pulses: 0, 6, 12, 18, 24, 30, 36, 42, 48, 54, and 60. (B ) Use-dependent block of Na+currents for tonicaine and QX-314. Use-dependent inhibition was elicited by repetitive pulses to +30 mV for 24 ms at a frequency of 2 Hz for a total of 60 pulses with the cell held at −100 mV (the pulse protocol is illustrated in A ). The concentrations used were equal to the 50% inhibitory concentrations of tonicaine and QX-314 (7 and 18 μm, respectively). Tonicaine showed a significantly greater additional use-dependency than QX-314. After 60 pulses, the peak amplitude for tonicaine still was decreasing, as opposed to QX-314 and the control, which were able to reach steady state inhibition before the sixtieth pulse.