Article Figures & Data
Figures
- Figure 1.
Prominent ADP following action potentials at MFBs in mouse hippocampal slices. A, IR-DIC image of the recorded bouton (arrow) and CA3 pyramidal cell (arrowhead). Scale bar represents 5 µm. B, Whole-cell recordings confirmed typical membrane potential responses of MFBs with single action potentials in response to the current injection (–20- to 120-pA pulses for 500 ms, 20 pA each step). C, Action potential of MFBs evoked by input stimuli at the granule cell layer (left) and current injection at the recorded boutons (right). The arrowheads represent the peaks of ADP. D, Summary data of the amplitude of ADP elicited by input stimuli (open column; n = 20) or by current injection (closed column, n = 6). E, Effects of Ca2+-containing ACSF on action potentials elicited by current injection. F, Summary of ADP amplitude in the absence (open column) and presence (closed column) of Ca2+ (n = 8).
- Figure 2.
Dependence of ADP on initial membrane potentials. A, Action potentials were recorded at control (black) and depolarized (red) membrane potentials by injecting constant currents into the recorded boutons. Although the peak of action potentials was unaffected by changes in initial membrane potentials, the amplitude and the time course of ADP were significantly altered by changes in the membrane potentials. B, Superimposed traces illustrate the voltage-dependence of ADP. C, Summary data for ADP amplitude recorded at control and depolarized initial membrane potentials (n = 14, *p < 0.05).
- Figure 3.
Involvement of Na+ channels in axonal ADP. A, Action potentials evoked by input stimuli (black) were completely suppressed by application of 0.5 µm TTX (red). Large current injection into the recorded MFB elicited mock action potentials with similar time courses of action potentials and ADP (blue). B, Superimposed control (black) and mock action potentials (blue) demonstrated that TTX-sensitive components overtook the TTX-resistant slow ADP, suggesting that TTX-sensitive Na+-channels partly enhance ADP. C, D, Comparison of control and mock action potentials (C) and ADP (D) amplitude (n = 6, *, p < 0.05). E, F, Effects of focal application of 1 µm veratridine, an inhibitor of Na+ channel inactivation, on ADP. Veratridine enhanced and prolonged ADP (E). In some cases, multiple action potentials (F) were overlaid, as shown in the right panel. G–I, Summary data for the effects of veratridine on the amplitude (G), half-width (H) of action potentials, and the amplitude of ADP (I; n = 6, *, p < 0.05).
- Figure 4.
Contribution of capacitive slow discharge in the axonal ADP. A, Mock action potentials (black) elicited by large current injection into MFBs in the presence of TTX were prolonged by the addition of 2 mm 4-AP (red), leaving the capacitive components of the axonal membrane. B, C, Summary data for the amplitude (B) and decay time constant (C) of ADP (n = 8, *, p < 0.05). D, Time constant of MFBs measured by hyperpolarizing current injection (–10 pA, 300 ms). E, Comparison of decay time course of capacitive components at different initial membrane potentials of –80 and –70 mV. F, Summary data for decay time constants of hyperpolarization (D) and the capacitive component of ADP at –80 mV (E). G, Simulated membrane potentials in the mossy fiber model by Engel and Jonas (2005) in response to hyperpolarizing current injection into distal MFBs (left). Brief large current injection elicited a similar response to those observed in A (right) when gNa and gK were omitted from the distal axons shown in red. See Methods for details. H, Simulation of passive propagation of upstream action potentials to distal axons where gNa and gK were omitted.
- Figure 5.
Broadening of axonal action potentials by the preceding ADP. A, Effects of changes in initial membrane potentials on action potentials elicited by stimulation of input fibers recorded at depolarized (red) or resting (black) membrane potentials. B, Summary data for the relative peak, half-width, and latency of action potentials (n = 14). C, Superimposed traces of paired-pulse responses at 10-, 20-, 50-, and 100-ms intervals. D, Summary data for the paired-pulse ratio of action potential amplitude measured from the onset of the second action potential (n = 8). E, First derivatives calculated from paired-pulse responses are shown in C and F, Summary data for the paired-pulse ratio of the first derivative waveform (n = 8). G, Expanded traces of the first (black) and second (red) action potentials at different interspike intervals. H, Summary data for the paired-pulse ratio of the action potential half-width (n = 8).
- Figure 6.
Facilitation of presynaptic Ca2+ influx by the axonal ADP. A, Ca2+ currents in MFBs evoked by voltage steps from –80 mV to –70∼50 mV for 20 ms. B, The averaged I-V relationships of presynaptic Ca2+ currents at MFBs. Current amplitudes were measured at the end of the voltage steps. C, Voltage-dependence of activation of Ca2+ currents as shown by the amplitudes of tail currents. Data were normalized to the maximal value in each experiment and were fitted using a Boltzmann function (n = 7). D, Voltage-clamp recording of Ca2+ currents using voltage commands mimicking the action potential waveforms elicited by paired-pulse stimuli (red). For comparison, mock action potential–like voltage commands mimicking broadening of action potentials by 10% in accordance with data in Fig. 5H, but lacking ADP, were applied (blue). Superimposed Ca2+ currents evoked by the first and second voltage commands are shown in the right panels. E, F, Summary data for the paired-pulse ratio of charge transfer (E) and half-width (F) of Ca2+ currents (n = 7, *, p < 0.05).
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