Activity-dependent broadening and static analog modulation of AP duration in axons of CA1 pyramidal neurons.

(A) Axonal recording from a CA1 pyramidal neuron. Confocal stack maximum projection of a CA1 pyramidal neuron filled with 100 µM Alexa Fluor 488. The somatic recording pipette is illustrated schematically. Axonal recording pipettes contain 100 µM Alexa Fluor 647. (B) Left, APs evoked by a train of short pulses applied to the soma (5-ms duration, 1.5 nA amplitude, 50 Hz). Black traces indicate somatic voltage and corresponding current, blue traces represent axonal voltage and corresponding current. Right, traces show first action potential in the train on an expanded timescale. Axonal recording site is 303 µm from the soma. (C) Overlay of 1–5th, and 50th axonal AP in the train evoked by brief current pulses applied to the soma. The somatic membrane potential before and after train stimulation was –60 mV. Axonal recording site is 303 µm from the soma. (D, E) Plot of normalized AP half-duration against stimulus number for proximal (≤200 µm; D) and distal (200–600 µm; E) axonal recordings at 5 (triangles, n = 6, and 6, respectively), 10 (squares, n = 8, and 8, respectively), and 50 Hz (circles, n = 13, and 18, respectively) stimulus frequency. Note significant extent of single-trial broadening after the first AP. (F) Overlay of the first axonal AP evoked by a brief current pulse applied to the soma. The somatic membrane potential preceding the stimulation was –50, and –60, respectively. Axonal recording site is 303 µm from the soma. (G) Summary bar graph of axonal AP durations at half-maximal amplitude of two different groups (–60 mV, black, or –50, red) pooled for proximal (≤200 µm, n = 13) and distal (200–600 µm, n = 18) axonal recordings. Bars indicate mean ± SEM; circles denote individual experiments. Data from the same experiment were connected by lines. **P<0.01, ***P<0.001, ****P<0.0001. (H) Axonal APs evoked at the soma 50 ms (thin) and 4.55 s (thick) after the onset of somatic depolarization (–50 mV). Plot of normalized AP half-width versus latency (Δt). Data fitted with a single exponential function (red line). Note the increase in AP half-width for long delays. (Inset) Stimulation protocol. Axonal recording site is ∼120 µm from the soma. (I) Plot of half-width of distal axonal AP against stimulus number. Data points for the same experimental conditions are connected by lines. Black squares, –60 mV; red circles, –50 mV. Error bars, SEM.