Paired whole-cell somatic and axon-attached recordings in neocortical PV-INs.

(A-B) Example (A) 2-photon z-stack and (B) morphologic reconstruction of a PV-IN in Layer 2/3 of mouse primary somatosensory cortex (soma and dendrites in black, axon in blue). The path from the axon hillock to the axonal recording site is indicated in orange. (C-D) Example somatic whole-cell voltage traces (top) and axon-attached current traces (bottom) during (C) brief depolarizing current pulses and (D) a depolarizing current step injected through the somatic pipette. (E) Plot of latency from the somatic to the axonal AP peak vs. the distance from the axon hillock to the axonal recording site (n = 32 cells from N = 23 mice).

PV-INs display use-dependent changes in the axonal AP waveform during brief depolarizing pulses.

(A) Experimental schematic for paired whole-cell somatic and axon-attached recordings in neocortical PV-INs during brief depolarizing pulses. (B-D) Examples of simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) traces during brief depolarizing pulses delivered at the soma pipette at (B) 50 Hz, (C) 100 Hz, and (D) 200 Hz. (E,G) Plot of axonal AP peak (normalized to the mean of the first 5 APs) vs. AP number for an example cell at (E) 100 Hz and (G) 200 Hz. Gray points indicate individual APs. The dark gray line indicates the mean across bins of 20 APs. (F,H) Summary data of axonal AP peak (normalized to the mean of the first 5 APs) vs. AP number at (F) 100 Hz and (H) 200 Hz. Gray lines indicate individual cells, the dark blue line indicates the mean across cells, and the shaded blue region indicates 95% confidence intervals (n = 25 cells from N = 14 mice).

Human fast spiking INs display use-dependent changes in the axonal AP waveform during brief depolarizing pulses.

(A) Experimental schematic for paired whole-cell somatic and axon-attached recordings in human neocortical fast-spiking INs during brief depolarizing pulses. (B) Morphologic reconstruction and example voltage trace in response to a square wave depolarization from a human fast-spiking aspiny presumptive basket cell. (C-D) Examples of simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) traces during brief depolarizing pulses delivered at the soma pipette at (C) 50 Hz and (D) 100 Hz.

Neocortical PV-IN axonal AP propagation fidelity remains high during physiologic AP trains.

(A) Experimental schematic of paired whole-cell somatic and axon-attached recordings in PV-INs (left) during delivery of a 5 min pulse train simulating in vivo PV-IN firing at the soma. (B) Example simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) traces during a physiologic AP train (right). (C-D) Axon AP peak (normalized to the mean of the first 5 AP peaks) vs. AP number in (C) an example cell and (D) as a summary across cells (dark blue indicates the mean, shaded region indicates the 95% confidence intervals; n = 8 cells from N = 4 mice).

PV-INs display progressive decreases in axonal AP propagation fidelity during seizure-like events induced by 50 µM bicuculline.

(A) Experimental schematic for paired whole-cell somatic and axon-attached recordings in neocortical PV-INs during seizure-like events induced by 50 µM bicuculline. (B) Example of a simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) trace from a PV-IN during spontaneous discharges within a seizure-like event. (C) Plot of axon AP peak (normalized to the mean of the first 5 AP peaks) vs. AP number during the final spontaneous discharge during the seizure-like event. (D) Raster plot of APs over time during the final spontaneous discharge of the seizure-like event. Black lines indicate the height of axon AP peaks (normalized to the mean of the first 5 axon AP peaks). Gray lines indicate the height of soma AP peaks (normalized to the height of the max soma AP peak) (n = 6 cells from N = 4 mice).

Use-dependent decreases in Ca2+ signal in PV-IN boutons.

(A) Experimental schematic for recordings of Fluo-5F imaging in neocortical PV-IN boutons during brief depolarizing pulses delivered at the soma. (B) Example image and Ca2+ traces from en passant boutons in the distal axon of a PV-IN. (C) Summary data of dG/R (change in green Fluo-5F fluorescence normalized to change in red fluorescence from non-functional fluorophore) vs. AP number during brief depolarizing pulses at 100 Hz (10 boutons from n = 5 cells from N = 3 mice). Light gray lines indicate traces from individual cells (with median filter smoothing), dark blue line indicates the mean across cells, light blue shaded region indicates 95% confidence intervals. (D) Quantification of dG/R values from (C) showing (D) peak dG/R values vs. mean across APs 2000-2500 and (E) mean across APs 500-1000 vs. mean across APs 2000-2500. Bars represent mean and standard error of the mean. P-values calculated using the Wilcoxon signed-rank test.

Use-dependent regulation of the axonal AP waveform can be modeled as a sum of exponential decays.

(A-D) Example plots of raw normalized axon AP peaks vs. time data for cells stimulated with (A) brief pulses at 100 Hz, (B) brief pulses at 200 Hz, (C) pulses at variable frequencies, and (D) square wave depolarizations. (E-G) Box and whisker plots of the (E) goodness of fit R2 (median = 0.76), (F) ∆A fitting parameter (median = 0.69e10-3), and (G) decay (τ) fitting parameter (median = 11.23 s) across cells. Data points colored by stimulation protocol (83 recordings from n = 52 cells from N = 35 mice). (H) Plot of axon AP peaks (normalized to the mean of the first 5 APs) vs. time for the simulated in vivo spike train data in Figure 4 (gray lines indicate individual cells, blue line indicates mean across cells) overlaid with axonal AP peaks predicted by the sum of exponential decays model parameterized with the median values from (F) and (G) (with outliers excluded).

High extracellular K+ can mimic use-dependent modulation of the axonal AP.

(A) Experimental schematic for paired whole-cell somatic and axon-attached recordings in neocortical PV-INs during brief depolarizing pulses while changing the concentration of extracellular K+. (B) Examples of simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) traces during brief depolarizing pulses delivered at the soma pipette at 100 Hz in ACSF with 2.5 mM K+, after wash-on of ACSF with 10 mM K+, and again after wash-on of 2.5 mM K+. (C-D) Summary data of axonal AP peak (normalized to the mean of the first 5 APs) vs. AP number during 100 Hz brief depolarizing pulses (C) at baseline in ACSF with 2.5 mM K+ and (D) after wash-on of ACSF with 10 mM K+. Gray lines indicate individual cells, while the dark black/orange line indicates the mean across cells with the shaded blue region indicating standard error of the mean (n = 7 cells from N = 4 mice). (E) Summary data of mean normalized axon AP peak for APs 750-1000 in 100 Hz pulse train in ACSF with 2.5 mM K+, after wash-on of ACSF with 10 mM K+ (p = 0.0156, Wilcoxon signed-rank test), and again after wash-on of 2.5 mM K+.

Use-dependent changes in PV-IN axonal AP waveforms outpace use-dependent changes in the somatic AP max dV/dt.

(A-B) Summary data of somatic AP max dV/dt (normalized to the mean of the first 5 APs) vs. AP number at (A) 100 Hz and (B) 200 Hz. Gray lines indicate individual cells, the dark magenta line indicates the mean across cells, and the shaded magenta region indicates 95% confidence intervals (n = 24 cells from N = 14 mice). Dark blue lines and shaded regions are a replication of the data presented in Fig. 2F,H for visual comparison.

PV-INs display use-dependent changes in the axonal AP waveform fidelity during square wave depolarizations.

(A) Experimental schematic for paired whole-cell somatic and axon-attached recordings in neocortical PV-INs during increasing square wave depolarizations. (B) Examples of simultaneous whole-cell somatic voltage (top) and axon-attached current (bottom) traces during square wave depolarizing pulses delivered at the soma pipette. (C-D) Plot of the mean of the last 5 axonal AP peaks in the current step (normalized to the mean of the first 5 APs in the recording) vs. firing frequency during the current step (C) for an example cell and (D) across multiple cells (gray lines indicate individual cells, dark blue line indicates mean across cells, shaded blue region indicates 95% confidence intervals; n = 33 cells from N = 23 mice). (E) Example trace of 20 Hz brief depolarizing pulses delivered after the current step, demonstrating that decreases in axonal AP peaks during current steps are not due to rundown in recording quality.

PV-IN axonal propagation depends on the distance and number of branch points traversed by the AP.

(A) Plot of the mean of APs 1500-1600 (normalized to the mean of the first 5 APs in the recording) vs. the distance from the axon hillock to the recording site for the data in Figure 2F (n = 33 cells from N = 23 mice). Solid line indicates linear regression (R2 = 0.25, p = 0.03). (B) Plot of the mean of APs 1500-1600 (normalized to the mean of the first 5 APs in the recording) vs. branch order for the recording site. Solid line indicates linear regression (R2 = 0.34, p = 0.01).

Barrages of ectopic APs are correlated with profound changes in the axonal AP waveform.

(A) Example trace of a paired whole-cell somatic and axon-attached recording in a neocortical PV-IN in response to square wave depolarizations in which the cell displayed spontaneous persistent firing of APs generated in the distal axon. (B) Example trace in which axonal AP propagation fidelity of distal axon-generated APs is higher at the distal axon recording site relative to the somatic recording site.

Use-dependent decreases in PV-IN-mediated synaptic transmission.

(A) Experimental schematic for paired somatic recordings of synaptic transmission in a pre-synaptic PV-IN and a post-synaptic Layer 2/3 pyramidal neuron. Pre-synaptic PV-INs were stimulated with 2ms pulses at 50, 100, or 200 Hz. (B-E) Examples of simultaneous whole-cell somatic voltage traces from a presynaptic PV-IN (top) and current traces from a post-synaptic Layer 2/3 pyramidal cell (bottom) during brief depolarizing pulses delivered to the pre-synaptic cell at (B) 50 Hz, (C) 100 Hz, and (D) 200 Hz. (B) displays the first 10 APs from (D) at higher temporal resolution. (F,H) Plot of IPSC peak (normalized to the amplitude of the first evoked IPSC) vs. AP number for an example cell at (F) 100 Hz and (H) 200 Hz. Gray points indicate individual APs, while dark gray line indicates the mean across bins of 25 APs. (G,I) Summary data of IPSC peak (normalized to the amplitude of the first evoked IPSC) vs. AP number at (G) 100 Hz and (I) 200 Hz. Gray lines indicate individual cells, while the dark blue line indicates the mean across cells and the shaded blue region indicates standard error (n = 3 cells from N = 3 mice).