Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex

Abstract

Most functional plasticity studies in the cortex have focused on layers (L) II/III and IV, whereas relatively little is known of LV. Structural measurements of dendritic spines in vivo suggest some specialization among LV cell subtypes. We therefore studied experience-dependent plasticity in the barrel cortex using intracellular recordings to distinguish regular spiking (RS) and intrinsic bursting (IB) subtypes. Postsynaptic potentials and suprathreshold responses in vivo revealed a remarkable dichotomy in RS and IB cell plasticity; spared whisker potentiation occurred in IB but not RS cells while deprived whisker depression occurred in RS but not IB cells. Similar RS/IB differences were found in the LII/III to V connections in brain slices. Modeling studies showed that subthreshold changes predicted the suprathreshold changes. These studies demonstrate the major functional partition of plasticity within a single cortical layer and reveal the LII/III to LV connection as a major excitatory locus of cortical plasticity.

Figure 1

The Time Course of Receptive Field Plasticity in Different Layers (Rat and Mouse) (A) Pattern of whisker deprivation. The D row whiskers were removed from D1 to D8. Delta and Gamma were left intact. This produced a single row of deprived barrels in the cortex. We recorded from the D1, D2, and D3 deprived barrel-related columns. (B) Poststimulus time histograms were constructed and analyzed for responses to stimulation of all whiskers immediately surrounding the principal whisker. Control: an example receptive field for a LV cell in an undeprived animal. Right: A typical LV receptive field in an animal deprived for 10 days. The shading indicates PSTHs corresponding to deprived whiskers. S1 and S2 refer to the largest and second largest responses respectively produced by surround whiskers. (St = whisker straddling the rows, i.e., gamma and delta). (C) Distribution of layer V responses to principal whisker (PW), S1 and S2 surround whiskers. For each deprivation length, response amplitude is plotted for each individual cell (black symbols) and the population distribution is shown at the side (shaded area, arrows: 20 cells for 0.05 spk/stim-wide bins). Red lines: response average versus time-course. (D) Time course of PW, S1, and S2 responses are plotted for rats (D1) and mice (D2) against the numbers of days of D-row deprivation. For rats, PW: note that LIV and LVa responses do not change over this period. LII/III PW responses are lower at 10 days but not 3 days. LVb responses decrease after 3 days deprivation. S1 and S2: LIV and LII/III responses do not change while LVa and Vb responses potentiate after 3 days and 10 days deprivation. See results for statistics. See also Tables S1 and S2.

Figure 2

In Vivo Classification of Pyramidal Cell Types (Rat) (A) Electrophysiological characterization. Left, examples of firing pattern in response to current injection used for classification. Right, quartile representation of distribution of LV electrophysiological parameters recorded during stimulation. Cell capacitance was calculated from double exponential fits of membrane response to negative current (10 pA, 0.2 Hz) injected in-between stimulation sequences. The minimum ratio of spike amplitude was calculated from evoked spike doublets. Kolmogorov-Smirnov test, ^∗∗p < 10⁻⁶. (B) Laminar distribution of the different cell types. Only LV IB and RS cells were considered in subsequent analysis. (C) Morphological analysis of IB and RS cells. (C1) Left, examples of biocytin-filled cells. Inset: zoom along the cellular soma for the same two cells (bar: 20 μm). Right: dendritic tree reconstruction for the two cells corresponding to the firing patterns presented in (A). (C2) Cell type analysis of morphologic parameters (mean ± SD). The width of the apical dendrite was calculated 10 μm away from the point where the dendrite emerges from the soma. Unpaired Student's t test, ^∗p < 0.05; ^∗∗p < 0.005. See also Figure S4.

Figure 3

Stimulation Methodology and Receptive Field Case Studies (Rat) (A) Pictures of the 3 × 3 whisker stimulation device. Each piezo is oriented at the natural angle of the whisker unless stimulated. (B) Schematic representation of one sequence of stimulation applied for recordings in the D2 column. For D1 or D3 recordings, the stimulators are translated so that the PW is always the central whisker. For each sequence the whiskers are stimulated with a dorsal deflection in a random order. An intracellular injection of current is made at the end of each sequence (0.1 pA, 100 ms). (C) Nine-whisker suprathreshold (PSTH) and subthreshold (wPSP) receptive fields for 4 example cells. Relative positions map the spatial arrangements of the whiskers. Vertical lines indicate stimulus onset. Black PSTH and wPSP correspond to the principal whisker (D2 in the four cases). The shaded areas highlight the responses to deprived whiskers stimulation. See also Figure S5.

Figure 4

Deprivation Induces Cell-Type-Specific Changes of LV Receptive Fields (Rat) (A) Left, schematic representation of the whiskers studied; right, illustration of the calculated parameters. (B) Suprathreshold receptive field. Principal whisker and two surround row-associated whiskers (T1 and T2) were trimmed, while six surround whiskers in the adjacent rows (S1 to 6) were left intact. Trimmed and spared surround whiskers were ordered respectively in decreasing order of suprathreshold response (mean ± SEM). Test: ANOVA, effect of deprivation on all spared or all trimmed whiskers; ^∗p < 0.05, ^∗∗p < 0.001. (C) Supra and subthreshold effects of deprivation on all trimmed whiskers averaged and all spared whiskers averaged. Each parameter is normalized to the control value. The subthreshold parameters (shaded areas) are calculated from the wPSPs. Receptive fields are detailed as in (B) for each parameter in Figure S1. Same statistics as in (B). See also Figures S1 and S5.

Figure 5

Temporal Properties of the Receptive Fields Are Differentially Affected by Deprivation (Rat) (A) IB cells response in control and deprived animals. Top: IB cell-population PSTH and wPSP for the principal and for the best spared whiskers in control and test animals. The average Vm after the no-stimulation event has been subtracted, and the baseline periods aligned. The difference between control and test wPSP is presented in gray. Dashed line: onset of stimulation. Bottom: Quantification of temporal parameters (mean ± SEM). The wPSP latency corresponds to the beginning of the depolarization, the a.p. latency is the averaged timing of the first spike following stimulation and the a.p. jitter is the standard deviation of the first spike timing. Unpaired Student's t test, ^∗p < 0.05. (B) RS cell population PSTH and wPSP for the principal and for the best spared whiskers in control and test animals. Conventions as in (A). See also Figure S2.

Figure 6

Measuring Synaptic Input Maps in Intrinsically Bursting and Regular Spiking Pyramidal Neurons (Mouse) (A) Top: Two biocytin-filled neurons in LVb (left, IB cell; right, RS cell). Bottom: Brightfield image of an acute brain slice showing the five rows of barrels. (B) Examples of dendritic morphologies (left, IB cell; right, RS cell) and the corresponding action potential firing patterns. (C) Quantitative morphology of the dendritic arbors of RS (n = 7) and IB (n = 7) cells. (D) First interspike interval (First ISI) versus membrane capacitance (solid circles, IB cell; open circles, RS cell). Cells with First ISI < 25 ms were classified as IB. (E) Example of a LSPS input map for a single LVb neuron (soma position is indicated by the triangle). Dashed boxes indicate barrel boundaries. The colored numbers in white circles indicate the locations of the UV stimulus generating the traces shown in (F). Pixels with direct responses are blacked out (see Experimental Procedures). (F) Examples of traces from the map shown in (E). 1, pure synaptic response; 2, direct response in the apical dendrite; 3, direct response in the perisomatic region. The gray zone represents the time used to score direct responses. See also Figures S3 and S4.

Figure 7

Plasticity of Synaptic Input Maps for RS and IB Cells in Deprived Barrel Columns (Mouse) (A and B) Average synaptic input maps of RS (n = 26) and IB (n = 46) cells under control conditions. The positions of barrels are indicated by dashed lines. The positions of the somata are indicated by white triangles. The areas in black indicate the regions where the number of traces not polluted by direct responses was too small and were excluded from analysis. (C and D) Average synaptic input maps of RS (n = 22) and IB (n = 33) cells in deprived barrel columns. The positions of spared (white) and deprived (red) barrels are indicated by dashed lines. (E and F) Horizontal spatial profiles of mean LII/III → LVB input to RS (E) and IB (F) cells in control (blue) and trimmed (red) animals. Dashed lines indicate the boundaries of the barrel columns. (G and H) Difference maps made by subtracting the averaged trimmed map (C and D) from the control map (A and B). (I) Horizontal spatial profiles of mean LII/III to LVb input to RS (open circles) and IB (filled circles) cells in control conditions. (J and K) Mean synaptic input sorted by layer of origin from the home (left) or surround columns (right) (indicated as gray boxes in the schematics) to RS (J) and IB (K) cells, in control (blue) and trimmed (red) animals. Background due to spontaneous synaptic currents is indicated by dashed lines. See also Figure S3.

Figure 8

Time Course of the LII/II to LVb RS Projection Center Depression and the LII/III to LVb IB Projection Side Potentiation (Mouse) (A and B) Averaged synaptic maps of RS (n = 13) and IB (n = 18) cells under control conditions. (C and D) In deprived barrels, 3 to 5 days after whisker trimming (RS: n = 10; IB:n = 13). (E and F) In deprived barrels 10 to 14 days after whisker trimming (RS: n = 13; IB:n = 16). (G) Time course of LII/III to LVb RS input from the home barrel (open circles), and LII/III to LVb IB input from the surround columns (filled circles), normalized to controls. See also Figures S3 and S8.