Fast-spiking interneurons fire in gamma frequency bursts, while stellate and pyramidal cells fire more sparsely.

A) Voltage traces from (i) stellate, (ii) pyramidal, and (iii) fast-spiking interneurons during optogenetic stimulation of CaMKIIα+ neurons. B) Raster plots showing 40 theta periods of sinusoidal 8 Hz optogenetic stimulation for the same recording as A. C) Spike probability histogram for each neuron relative to phase of theta stimulation [mean (solid line) ± S.E.M. (shaded region)]. Example spike probability from neurons shown in A and B (dashed line). D) Interspike firing rate histogram for each neuron [mean (solid line) ± S.E.M. (shaded region)]. Example spike probability from neurons shown in A and B (dashed line).

Fast-spiking interneurons receive strong excitation and moderate inhibition, while stellate and pyramidal cells receive strong inhibition.

A) Example post-synaptic currents recorded during optogenetic stimulation of CaMKIIα+ neurons. From top to bottom: EPSCs from fast-spiking interneuron. IPSCs recorded from stellate, pyramidal, and fast-spiking interneuron. B) Average scalogram of gamma-filtered PSCs from neurons in A. Left: EPSCs from fast-spiking interneuron. Right: IPSCs from pyramidal cell. C) Gamma frequency of PSCs from all neurons. Left: EPSCs from fast-spiking interneurons. Right: IPSCs from stellate, pyramidal, and fast-spiking interneurons. D) Peak gamma power of PSCs from all neurons. Left: EPSCs from fast-spiking interneurons. Right: IPSCs from stellate, pyramidal, and fast-spiking interneurons.

EPSCs precede IPSCs and LFP during theta frequency optogenetic stimulation of CaMKIIα+ neurons.

A) Example IPSCs received by stellate cell (red) and concurrent LFP (black). B) Gamma frequency filtered IPSCs (red) and LFP (black) from A. C) Average scalogram of gamma filtered IPSCs (red) and LFP (black) from B demonstrating similar gamma frequency oscillations. D) Average cross correlogram (black) between gamma filtered currents and LFP from B and individual theta cycles (gray). IPSCs are highly correlated with the LFP with no lag, demonstrating that the LFP oscillation is likely driven by local inhibitory currents. E) Average peak cross correlation coefficient between EPSCs and concurrent LFP. EPSCs received by fast-spiking interneurons are highly correlated with the LFP, indicating a strong PING mechanism. F) Average peak cross correlation coefficient between IPSCs received by stellate (red), pyramidal (green) and fast-spiking interneurons (blue) and the concurrent LFP. IPSCs are highly correlated with the LFP. G) Average peak correlation lag between EPSCs and LFP. The LFP lags the EPSCs by approximately 3 ms. H) Average peak correlation lag between IPSCs and LFP. The LFP is oscillating approximately at the same time as the IPSCs.

mEC neurons are phase locked to gamma oscillations.

A) Example voltage trace of a fast-spiking interneuron (blue) and concurrent LFP (black) during optogenetic stimulation of CaMKIIα+ neurons. B) Filtered gamma frequency LFP component (black) and raster plot of fast-spiking interneuron (blue) from A. C) Resultant vector comparison between stellate (red), pyramidal (green), and fast-spiking interneurons (blue). Left: Individual neurons. Right: Pooled within cell types. D) Pooled gamma spike phase distributions for stellate, pyramidal and fast-spiking interneurons.

Voltage imaging reveals network dynamics of layer II/III mEC neurons.

A) Example field of view for targeted illumination voltage imaging. Fluorescent neurons express Voltron2. B) Average LFP Scalogram demonstrates strong gamma frequency activity. C) Voltage traces from 41 neurons shown in A and the simultaneous LFP activity. Inset zooms in on traces within dashed rectangle. D) Average spike timing of all neurons in C (n = 41) relative to theta stimulation phase [mean (black) ± S.E.M. (gray)]. Average LFP gamma (blue) lags excitatory neuron spiking oscillations. E) Spike raster plot (black) of all neurons in C (n = 41) during 4 consecutive cycles of 8 Hz optogenetic stimulation. Light stimulus is shown in light blue. Average LFP (blue) and spike histogram [mean (black) ± S.E.M. (gray)] aligned with example theta period using dashed lines. F) Average interspike firing rate histograms per theta stimulation period from neurons in all imaging sessions (n = 240). G) Relationship between spike train correlations of all neurons (n = 240) and pairwise spatial distance. H) Comparison of spike train correlations for all neurons (n = 240) across different theta stimulation frequencies.

Local clusters of excitatory mEC neurons exhibit correlated voltage activity.

A) Raw voltage correlation matrix from 41 mEC neurons in Fig. 5. B) Agglomerative dendrogram of average voltage correlation linkages from pairwise correlations in A. C) Sorted voltage correlation matrix based on hierarchical clustering optimal leaf order. D) Sorted time series voltage activity based on sorted voltage correlation matrix. E) Spatial organization of voltage correlation clusters across the imaging field of view. F) Pairwise voltage correlations are weakly correlated with distance.

Simulation of PING mechanism during optogenetic theta drive of stellate cells.

A1) Voltage traces from PV-INs, A2) Voltage traces from SCs. A3) Downsampled raster plot showing SCs (red) and PV-INs (blue). B1) Top: Inhibitory synaptic currents (red) from a representative SC voltage-clamped at −70 mV. Bottom: Average scalogram of the inhibitory synaptic currents during a theta cycle. B2) Top: Excitatory synaptic currents (blue) from a representative PV-IN clamped at 0 mV. Bottom: Average scalogram of excitatory synaptic currents during a theta cycle. B3) Average cross-correlogram between gamma filtered IPSCs and EPSCs shows that the IPSCs received by the SCs follow the EPSCs recorded in the PV-INs by 3 ms.

mEC neurons are strongly phase locked to theta and moderately phase locked to gamma oscillations.

A) Pairwise theta phase consistency of stellate (red), pyramidal (green), and fast-spiking interneurons (blue) for each consecutive spike per theta stimulation period. All cell types and spike numbers are strongly phase locked to theta drive. B) Pairwise gamma phase consistency of stellate (red), pyramidal (green), and fast-spiking interneurons (blue) for each consecutive spike per theta stimulation period. The first spike per theta stimulation period is moderately phase locked to LFP gamma for all cell types. Fast-spiking interneurons are moderately phase locked to LFP gamma for up to 3 spikes per theta stimulation period.

Minimal PV+ overlap with Voltron2 expression in layer II/III mEC.

A) Example voltage imaging field of view with dense Voltron2 expression. B) Histological image of Voltron2-JF585 fluorescence registered to experimental field of view. C) Immunohistological image of PV expression in the same field of view as A and B. D) Composite image of Voltron2-JF585 (red) and PV (blue) expression in layer II/III mEC demonstrating minimal overlap between the populations.

LFP gamma frequency activity doesn’t change with theta stimulation frequency.

A) Average scalogram peak LFP gamma power from each voltage imaging field of view. B) Average scalogram LFP gamma frequency. C) Average scalogram LFP theta phase of peak gamma power.

Strong theta (left), but not gamma (right) phase locking across multiple consecutive spikes per theta period from neurons in all imaging sessions.

First spike gamma phase locking increases with theta stimulation frequency.

Spiking activity is not clustered across mEC.

A) Agglomerative dendrogram of average spike correlation linkages between 41 mEC neurons in Fig. 5, 6. B) Sorted spike correlation matrix based on hierarchical clustering optimal leaf order. C) Sorted time series voltage activity based on hierarchical clustering of spike correlation matrix. D) Spatial organization of spike correlation clusters across the imaging field of view. E) Pairwise spike correlations are not correlated with distance.