Parvalbumin-expressing interneurons linearly transform cortical responses to visual stimuli

Abstract

The response of cortical neurons to a sensory stimulus is shaped by the network in which they are embedded. Here we establish a role of parvalbumin (PV)-expressing cells, a large class of inhibitory neurons that target the soma and perisomatic compartments of pyramidal cells, in controlling cortical responses. By bidirectionally manipulating PV cell activity in visual cortex we show that these neurons strongly modulate layer 2/3 pyramidal cell spiking responses to visual stimuli while only modestly affecting their tuning properties. PV cells' impact on pyramidal cells is captured by a linear transformation, both additive and multiplicative, with a threshold. These results indicate that PV cells are ideally suited to modulate cortical gain and establish a causal relationship between a select neuron type and specific computations performed by the cortex during sensory processing.

Figure 1. Distinct Visual Response Properties of PV Cells versus Pyr Cells

(A) Left: configuration for two-photon targeted loose-patch recordings in mouse V1; PV cell expressing tdTomato targeted for recording with green electrode. Right: spiking response of the same PV cell to each of 12 directions of a drifting sinusoidal grating (2 s duration, shaded gray box). (B) Raster plots and peristimulus time-histograms (PSTHs) illustrate the cell's spiking response to repeated grating presentations. Same cell as in (A). (C) Top: tuning curve, i.e., the average spike-rate during the stimulus presentation of the 12 grating directions, illustrated both on Cartesian and on polar axes (same cell as A). The tuning curve plotted on Cartesian axes is fitted with a double Gaussian. The cell's spontaneous firing rate is illustrated by the dotted line. The cell's poor modulation as a function of orientation is reflected in a low orientation selectivity index (OSI). Bottom: spiking response of a different PV cell as a function of contrast, fit with a hyperbolic ratio function (line). Error bars are the standard error of the mean (SEM). (D) Top left: histogram of visually evoked spike rates (PV: n = 79; Pyr: n = 80). Top right: histogram of OSI (PV: n = 63; Pyr: n = 60). Inset: median PV and Pyr cell tuning curves normalized to the peak and fit with double Gaussian (shaded areas illustrated the 2^nd and 4^th quartile). Note that Pyr cells have a far greater OSI than PV cells. Bottom left: histogram of tuning sharpness, measured as the half-width at half-height (HWHH) of the double Gaussian fit (PV: n = 63; Pyr: n = 60). Inset: median PV and Pyr cell HWHH. Bottom right: histogram of contrast exponent, n, of hyperbolic ratio fit to contrast response, (PV: n = 43; Pyr: n = 30). Inset: mean PV and Pyr cell contrast response and fit (shaded area is the SEM). (E) Raster plots and PSTH illustrate the spiking response of a Pyr cell to 12 directions of oriented grating stimuli. (F) Top: tuning curve of the cell in (E). Note the cell's high OSI as compared to the PV cell in (C). The cell's spontaneous firing rate is illustrated by the dotted line. Bottom: spiking response of a different Pyr cell as a function of contrast, fit with the hyperbolic ratio function (line). Error bars are the SEM.

Figure 2. Bidirectional Modulation of PV Cell Activity Demonstrates Their Tight Control of Pyr Cell Contrast Response

(A) Top: coronal section of V1 from a PV-Cre × tdTomato reporter mouse injected with Cre depend Arch-GFP AAV-vector. In red: the naive expression of tdTomato; in green: Arch-GFP expressing cells (anti-GFP immunostaining). Bottom: image detail. Note that Arch-GFP expression only occurs in tdTomato expressing, and hence PV expressing, cells. (B) Left: PV cell expressing Arch-GFP targeted for loose-patch recording with red electrode. Right: raster plot and PSTH of spiking response to grating during interleaved trials: either control (cyan, no led) or during PV cell suppression by Arch photo stimulation (green, LED on). Gray area on the raster plot illustrates the duration of the visual stimulus; shaded area on the PSTH is the bootstrapped 95% confidence interval (cyan and green). LED illumination is illustrated with horizontal blue bar. Note that the time course of PV cell suppression tightly matches the time course of LED illumination. (C) Left: PV cell expressing Chr2-tdTomato targeted for loose-patch recording with green electrode. Right: raster plot and PSTH of spiking response to grating during interleaved trials: either control (cyan, no LED) or during PV cell activation with ChR2 photo stimulation (red, LED on). Shaded area and horizontal bar as in (B). Note that the time course of PV cell inactivation tightly matches the time course of led illumination. (D) Left: example of contrast response of a single PV cell in control (cyan) and during suppression by Arch photo stimulation (green). Error bars are the SEM. Right, gray: visually evoked spike rate of PV cells under control conditions versus during Arch photo stimulation (circles; 8 different contrasts for each cell; n = 14 cells) Lines: linear fits to each cell; green line: average linear fit; dotted line at unity. (E) Left: example of contrast response of a single PV cell in control (cyan) and during activation with ChR2 (red). Error bars are the SEM. Right, gray: visually evoked spike rate of PV cells under control conditions versus during ChR2 photo stimulation (circles; 8 different contrasts for each cell; n = 11 cells). Lines: linear fits to each cell; red line: average linear fit; dotted line at unity. (F) Left: contrast response of a single Pyr cell in control (black) and during suppression of PV cells (green). Error bars are the SEM. Right, gray: visually evoked spike rate of Pyr cells under control conditions versus during PV cell suppression (circles; 8 different contrasts for each cell; n = 17 cells). Lines: linear fits to each cell; green line: average linear fit; dotted line at unity. (G) Left: example of contrast response of a single Pyr cell in control (black) and during activation of PV cells (red). Error bars are the SEM. Right, gray: visually evoked spike rate of Pyr cells under control conditions versus during PV cell activation (circles; 8 different contrasts for each cell; n = 10 cells). Lines: linear fits to each cell; red line: average linear fit; dotted line at unity.

Figure 3. Slight Modulation of Pyr Cell Tuning Properties by PV Cells

(A) Left: PSTH of Pyr cell response to drifting gratings during control (black) or Arch-mediated suppression of PV cells (green). Horizontal bars: black, stimulus presentation; blue, LED illumination. Right: the tuning of the cell is illustrated on polar (top) and Cartesian (middle) axes (black: control; green: PV cell suppression). The tuning curves plotted on Cartesian axes are fitted with a double Gaussian. Note the slight reduction in both orientation and direction selectivity indexes. Bottom: the Gaussian fits under control (black) and PV cell suppression (green) are normalized to the peak and superimposed for comparison. Error bars are the SEM. Note that the tuning sharpness during PV cell suppression remains essentially unaffected. (B) Left: PSTH of Pyr cell response to drifting gratings during control (black) or ChR2-mediated activation of PV cells (red). Horizontal bars: black, stimulus presentation; blue, LED illumination. Right: the tuning of the cell is illustrated on polar (top) and Cartesian (middle) axes (black: control; red: PV cell activation). The tuning curves plotted on Cartesian axes are fitted with a double Gaussian. Note the increase in both orientation and direction selectivity indexes. Bottom: the Gaussian fits under control (black) and PV cell activation (red) are normalized to the peak and superimposed for comparison. Error bars are the SEM. Note that the tuning sharpness during PV cell activation remains essentially unaffected. (C) Linear regression (line) of percentage change in Pyr cells firing rate at their preferred orientation versus the change in their tuning properties during PV cell photo suppression (green dots; n = 31) or PV cell photo activation (red dots; n = 14). p values (black) indicate the significance of correlation between in change firing rate and tuning property. Left: distributions of respective tuning properties (negative values indicate decrease in tuning property; p value indicates confidence that there is a significant difference; green: Arch versus control; red: ChR2 versus control). Bottom: distribution of change in firing rate. Arrows point to population medians.

Figure 4. A Linear Function with a Threshold Describes How PV Cells Transform Pyr Cell Responses

(A) Median response of Pyr cell population to 12 directions under control conditions (black circles) and upon PV cell photo suppression (green circles; n = 31) or PV cell photo activation (red circles; n = 14). Error bars are bootstrapped 95% confidence intervals. Black solid lines: double Gaussian fits to median control response. (B) Median response of Pyr cell population to 12 directions during PV cell photo suppression (green circles) or PV cell photo activation (red circles) plotted against control conditions (same data as A). Solid lines: threshold-linear function fit to data (green: slope of 1.2 and offset of 11%; red: slope 0.7 and offset of −13%). Gray dashed line extends the red linear fit to negative Pyr cell firing rates (i.e., the linear function without applying the threshold). Gray solid line is at unity. (C) Same data as (A), with the addition of the quantitative prediction of Pyr cell tuning curves (green and red lines) based on the threshold-linear functions in (B). Note the threshold-linear functions illustrates that activation/suppression of PV cells linearly scales Pyr cell responses regardless of stimulus orientation except where the responses are pushed below zero (gray lines). As a result, there is no change in HWHH and a small change in orientation and direction selectivity.

Figure 5. A Model Based on Experimentally Determined Synaptic Conductances Captures Linear Transformation of Pyr Cell Responses

(A) Left: visually evoked inhibitory postsynaptic currents (IPSC) recorded in a Pyr cell during control (cyan) and PV cell suppression (green). Average of 38 sweeps at each condition. Horizontal bars: black, stimulus presentation; blue, LED illumination. Brackets: black, baseline; gray, time over which average IPSC amplitude is computed. Right: scatter plot of visually evoked inhibitory conductance in control versus during PV cell photo suppression (open circles); “X” marks individual cells with significant change in conductance. Solid circle illustrates mean. Average decrease ∼10%; n = 13; p < 0.03. (B) Left: visually evoked excitatory postsynaptic currents (EPSC) recorded in a Pyr cell during control (red) and PV cell photo suppression (green). Average of 62 sweeps at each condition; horizontal bars and brackets as above. Note that while there is no change in the visually evoked EPSCs relative to baseline (black bracket), a significant increase in EPSC amplitude occurred systematically at led onset (asterisk; 0.1 ± 0.02 nS; n = 10; p < 0.004). Right: scatter plot of visually evoked excitatory conductance in control versus during PV cell photo suppression (open circles); “X” marks individual cells with significant change in conductance; solid circle illustrates mean. Excitatory conductance did not change significantly; n = 10; p = 0.5). (C) Top: visually evoked IPSCs (cyan) and EPSCs (red) recorded in a layer 2/3 Pyr cell during the presentation of six sinusoidal grating orientations. Bottom, dots: summary of excitatory (red; n = 4) and inhibitory (cyan; n = 5) tuning as a function of orientation. Error bars are the SEM. Lines are the respective Gaussian fits. (D) Tuning of excitatory synaptic conductance (red) and inhibitory synaptic conductance (cyan: control; green: during PV cell suppression, i.e., 10% reduction) as a function of orientation. Lines are the Gaussian fits from (C). (E) Net depolarization in the membrane potential of modeled cell (resulting from conductances in D) as a function of orientation under control conditions (black) and during PV cell suppression (green). The dotted line illustrates the spike threshold. Note that under control conditions the membrane potential is above threshold at most orientations. (F) Model cell's orientation tuning, i.e., firing rate as a function of orientation under control conditions (black; OSI = 0.67; HWHH = 24 degrees) and during PV cell suppression (green; OSI = 0.59; HWHH = 26 degrees). Inset, left: the expansive nonlinear threshold or power law, i.e., the firing rate as a function of net membrane potential depolarization. Inset, right: orientation tuning curves in normalized to the peak. A 10% decrease in inhibition, as experimentally determined, results in ∼50% increase in spiking response at the preferred orientation, a modest decrease in OSI (ΔOSI = 0.08), and a negligible change in tuning sharpness (ΔHWHH = 2 degrees).