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. 2016 Jul 26;16(4):1005-1015.
doi: 10.1016/j.celrep.2016.06.057. Epub 2016 Jul 14.

Cell Type-Specific Differences in Spike Timing and Spike Shape in the Rat Parasubiculum and Superficial Medial Entorhinal Cortex

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Free PMC article

Cell Type-Specific Differences in Spike Timing and Spike Shape in the Rat Parasubiculum and Superficial Medial Entorhinal Cortex

Christian Laut Ebbesen et al. Cell Rep. .
Free PMC article

Abstract

The medial entorhinal cortex (MEC) and the adjacent parasubiculum are known for their elaborate spatial discharges (grid cells, border cells, etc.) and the precessing of spikes relative to the local field potential. We know little, however, about how spatio-temporal firing patterns map onto cell types. We find that cell type is a major determinant of spatio-temporal discharge properties. Parasubicular neurons and MEC layer 2 (L2) pyramids have shorter spikes, discharge spikes in bursts, and are theta-modulated (rhythmic, locking, skipping), but spikes phase-precess only weakly. MEC L2 stellates and layer 3 (L3) neurons have longer spikes, do not discharge in bursts, and are weakly theta-modulated (non-rhythmic, weakly locking, rarely skipping), but spikes steeply phase-precess. The similarities between MEC L3 neurons and MEC L2 stellates on one hand and parasubicular neurons and MEC L2 pyramids on the other hand suggest two distinct streams of temporal coding in the parahippocampal cortex.

Figures

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Figure 1
Figure 1
Parasubicular and Superficial Medial Entorhinal Cortex Neuron Types (A) Top: tangential section of the parasubiculum (PaS) and layer 2 of the medial entorhinal cortex (MEC) stained for calbindin (Cb, green channel) and wolframin (WFS1, red channel). Bottom: parasagittal section of the MEC stained for Cb (green channel) and PCP4 (red channel). Also visible are the presubiculum (PrS) and postrhinal cortex (Por). (B) Left: reconstructions (from tangential cortical sections; neurons are seen from the top) of examples of the four neuron types: a PaS neuron (blue), an MEC L2 pyramidal neuron (green), an MEC L2 stellate cell (black), and an MEC L3 neuron (red), corresponding to the anatomical cell types marked by arrows in (A). Right: juxtacellular recording traces of the reconstructed cells. The spiking of the parasubicular neuron and the MEC L2 pyramid is bursty and theta-modulated. Scale bars, 1 mV. Cell reconstructions were adapted from Tang et al., 2014a, Tang et al., 2015, Tang et al., 2016.
Figure 2
Figure 2
Classification of Bursty and Non-bursty neurons (A) Example ISI distribution of a bursty (left) and non-bursty (right) juxtacellularly recorded neuron (bin width, 2 ms). (B) Top: scatterplot of the first two principal components (PC1 and PC2) obtained from a PCA of ISI distributions (black dots). The neurons form a C-shaped structure, as described by Latuske et al. (2015) (2D kernel smoothed density estimate indicated by lines). Bottom: the first three PCs of the ISI histograms. (C) Top: 3D scatterplot of the first three PCs, assigned to two clusters using a k-means clustering algorithm. Center-of-mass of bursty neurons (orange) and non-bursty neurons (purple) are indicated by black crosses. Bottom: projection of ISI distributions onto the optimal linear discriminant (the burstiness) of the two clusters revealed a bimodal distribution of bursty (orange) and non-bursty (purple) neurons. (D) Left: ISI histograms of all classified neurons, sorted by burstiness (scaled to maximum probability for each neuron for visibility). Right: example ISI histograms of neurons at the edges and in the middle of the clusters. Bursty neurons tend to fire burst at 125–250 Hz (4- to 8-ms intervals).
Figure 3
Figure 3
Burstiness in the Parasubiculum and Superficial Medial Entorhinal Cortex (A) Median ISI histogram (bin width, 2 ms) of all neurons recorded in the PaS (blue), identified and putative MEC L2 pyramidal neurons (green), identified and putative MEC L2 stellate cells (black), and MEC L3 neurons (red). Grey lines indicate 95% confidence intervals of the median. (B) Comparison of the proportions of the numbers of bursty (orange) and non-bursty (purple) neurons for the four different neuron types defined in (A). White areas denote cells that fall in the ambiguous zone between non-bursty and bursty (χ2 tests of equal proportions among cell types). (C) Comparison of the burstiness for the four different neuron types defined in (A). Vertical lines indicate medians (Mann-Whitney U tests).
Figure 4
Figure 4
Spike Shapes in the Parasubiculum and Superficial Medial Entorhinal Cortex (A) Peak-aligned and voltage-scaled spike shapes of cells in the PaS (blue), identified and putative MEC L2 pyramidal neurons (green), identified and putative MEC L2 stellate cells (black), and MEC L3 neurons (red). (B) Left: mean spike shapes of the four neuron types in (A) show differences in peak-to-trough time. Right: close-up of the trough of the mean spike shapes. (C) Comparison of peak-to-trough times of neurons as defined in (A) (Mann-Whitney U test; horizontal lines indicate means).
Figure 5
Figure 5
Theta Rhythmicity and Theta Cycle Skipping in the Parasubiculum and Superficial Medial Entorhinal Cortex (A) Example ISI histograms (black bars) of non-rhythmic (left), rhythmic and non-skipping (middle), and rhythmic but theta cycle-skipping (right) juxtacellularly recorded neurons. Solid red lines show maximum likelihood estimates of the ISI, and dashed blue lines indicate a flat model (no rhythmicity or cycle skipping). Bin width, 1 ms. (B) Flow diagram of the cell classification procedure. First we checked for rhythmicity and then for cycle skipping. (C) Left: comparison of the proportions of non-rhythmic and rhythmic neurons recorded in the PaS, identified and putative MEC L2 pyramidal neurons, identified and putative MEC L2 stellate cells, and MEC L3 neurons. Right: comparison of the proportions of rhythmic, non-cycle-skipping and rhythmic, theta cycle-skipping neurons recorded in the four neuron types. The generally rhythmic cell types (PaS and Pyr) have a larger proportion of theta cycle-skipping neurons than the generally non-rhythmic cell types (Stel and L3).
Figure 6
Figure 6
Phase Precession Slopes and Ranges in the Parasubiculum and Superficial Medial Entorhinal Cortex (A) Detection of single runs. Top: firing rate (red line) is estimated by convolving spikes (blue ticks) with a Gaussian kernel. Detected runs are indicated by gray shading. Bottom: theta phase of spikes as a function of time (black dots). Phase precession slopes and ranges of single runs are estimated by circular-linear fits (dashed lines). (B) Temporally defined single runs (black lines) match regions of elevated firing rate (color coded). Data are from the neuron shown in (A). (C) Examples of single-run phase precession for parasubicular (blue dots), identified MEC L2 pyramidal (green dots), identified MEC L2 stellate (black dots), and MEC L3 (red dots) neurons. Each dot represents the theta phase angle of a spike as a function of time. Dashed lines depict circular-linear fits. (D) Median single-run phase precession slopes for the four neuron types defined in (C). Single-run slopes are significantly larger in MEC L2 stellate and MEC L3 neurons than in parasubicular and MEC L2 pyramidal neurons (whiskers indicate 95% confidence intervals of the median). (E) Median single-run phase precession ranges among the four neuron types as defined in (C) and (D). Single-run phase ranges are significantly larger in MEC L2 stellate and MEC L3 neurons than in parasubicular and MEC L2 pyramidal neurons (whiskers indicate 95% confidence intervals of the median).

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