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. 2010 Nov 17;30(46):15479–15490. doi: 10.1523/JNEUROSCI.3982-10.2010

Figure 3.

Figure 3.

Presynaptic Na+ current generates the rising phase of the DAP. a, Spikes evoked by afferent fiber stimulation in the absence (control; black trace) and presence of external TTX (20 nm, gray traces during onset of application and red traces end of application). For a pair of APs, with 4 ms stimulus interval, removing the DAP (arrowhead) delayed the latency to the peak of the second AP (red arrow). b, After the complete inhibition of voltage-dependent Na+ currents with 1 μm TTX, brief current injections [1.5 ms duration, 0.5 nA (red trace) or 1 nA (black trace)] depolarized the calyx to 0 mV (red) or +30 mV (black). Note the absence of a DAP after the larger spike, whereas the smaller spike has a monotonically decaying DAP with no hump. c, P13 calyx AP recorded with a pipette solution containing a low concentration of QX-314 (0.2 mm), a Na+ channel blocker. The black trace was obtained within a few seconds after break-in, and the red trace was recorded after 7 min of whole-cell recording. After 3 min of QX-314 dialysis, the rising phase of the DAP was significantly inhibited, although spike amplitude was little changed (gray trace). d, The calyx of Held was dialyzed for 5 min with QX-314 (0.3 mm), and the stunted rise of the DAP was recorded (red trace). The recording pipette was then removed, and the same calyx was patched again using normal internal pipette solution without QX-314. The normal rise and amplitude of the AP spike and DAP recovered after 7 min of dialysis, which removed QX-314 from the calyx through the patch pipette. Note the broader AP spike with QX-314 (inset). The removal of QX-314 also recovered the second AP, which failed to fire in the presence of QX-314. e, Veratridine (an activator of Na+ channels; 1 μm) increased significantly the amplitude of the DAP after 1 min (red trace) and elicited repetitive spontaneous spiking in response to a single afferent fiber shock (blue trace; recorded 10 s after red trace). f, Summary of DAP amplitude in the presence of TTX (20–50 nm, n = 5), QX-314 (0.2–0.3 mm, n = 7), and veratridine (1 μm, blue triangles, n = 3). Data were analyzed with paired t test (**p < 0.01; ***p < 0.001).