Parvalbumin-expressing interneurons can act solo while somatostatin-expressing interneurons act in chorus in most cases on cortical pyramidal cells

Abstract

Neural circuits in the cerebral cortex consist primarily of excitatory pyramidal (Pyr) cells and inhibitory interneurons. Interneurons are divided into several subtypes, in which the two major groups are those expressing parvalbumin (PV) or somatostatin (SOM). These subtypes of interneurons are reported to play distinct roles in tuning and/or gain of visual response of pyramidal cells in the visual cortex. It remains unclear whether there is any quantitative and functional difference between the PV → Pyr and SOM → Pyr connections. We compared unitary inhibitory postsynaptic currents (uIPSCs) evoked by electrophysiological activation of single presynaptic interneurons with population IPSCs evoked by photo-activation of a mass of interneurons in vivo and in vitro in transgenic mice in which PV or SOM neurons expressed channelrhodopsin-2, and found that at least about 14 PV neurons made strong connections with a postsynaptic Pyr cell while a much larger number of SOM neurons made weak connections. Activation or suppression of single PV neurons modified visual responses of postsynaptic Pyr cells in 6 of 7 pairs whereas that of single SOM neurons showed no significant modification in 8 of 11 pairs, suggesting that PV neurons can act solo whereas most of SOM neurons may act in chorus on Pyr cells.

Figure 1

Unitary IPSC (uIPSC) of Pyr cells evoked by action potentials of PV or SOM neurons in vivo. (a) A PV neuron generated action potential which induced uIPSC in a Pyr cell, as indicated by arrows. Arrangement of the two cells and electrodes is shown in inset. The distance between the soma of the two cells was 26 μm. The cell bodies and dendrites are visualized by Alexa. Green signals mostly represent dendrites of other PV cells which expressed ChR2. Scale at bottom, 20 μm. (b) A SOM neuron generated action potential which induced uIPSC in a Pyr cell, as indicated by arrows. Other conventions are the same as in (a). (c–f) Peak amplitude, rising slope, decay tau and total charge of uIPSCs of 8 Pyr cells evoked by action potentials of PV neurons (left) and those of another 8 Pyr cells evoked by action potentials of SOM neurons (right). Filled symbol with vertical bar in the right represents means ± SEM. In a few cases the value of SEM was very small so that it did not appear outside the symbol. Triple asterisks indicate that the difference in the mean between the left and right columns is statistically significant at p < 0.001 (unpaired t-test).

Figure 2

Population IPSC (pIPSC) evoked by photoactivation of a mass of PV or SOM neurons. (a,b) pIPSCs recorded from the same neurons as in Fig. 1a and b, respectively, so that the traces are superimposed with the uIPSCs shown in Fig. 1 at the slower time scale. (c–f) Peak amplitude, rising slope, decay tau and total charge of pIPSCs of 12 Pyr cells evoked by the photoactivation of PV neurons (left) and those of another 11 Pyr cells evoked by the photoactivation of SOM neurons (right). Double and single asterisks indicate that the difference in the mean between the left and right columns is statistically significant at p < 0.01 and p < 0.05, respectively (unpaired t-test).

Figure 3

Ratio of the total charge of pIPSC (pIPSQ) to that of uIPSC (uIPSQ). Each symbol represents the ratio value for a pair of PV → Pyr cells (left column) and of SOM → Pyr cells (right column) obtained from the in vivo cortex. Filled symbol with vertical bar in the right represents means ± SEM. In the left column the value of SEM was very small so that it did not appear outside the symbol. Double asterisks indicate that the difference in the mean between the left and right columns is statistically significant at p < 0.01 (unpaired t-test).

Figure 4

uIPSCs of Pyr cells evoked by action potentials of PV or SOM neurons in vitro. (a,b) uIPSCs superimposed with action potential of a presynaptic PV and SOM neurons, respectively, recorded from Pyr cells in the slice preparation. Other conventions are the same as in Fig. 1a and b. (c–f) Peak amplitude, rising slope, decay tau and total charge of uIPSCs evoked by action potentials of PV neurons (left) and those evoked by action potentials of SOM neurons (right). Triple and single asterisks indicate that the difference in the mean between the left and right columns is statistically significant at p < 0.001 and p < 0.05, respectively (unpaired t-test).

Figure 5

pIPSCs evoked by photoactivation of a mass of PV or SOM neurons in slice preparations. (a,b) pIPSCs recorded from the same Pyr cells as in Fig. 4a and b, respectively, so that the traces are superimposed with the uIPSCs shown in Fig. 4, respectively. (c–f) Peak amplitude, rising slope, decay tau and total charge of pIPSCs of 15 Pyr cells evoked by the mass activation of PV neurons (left) and those of another 13 Pyr cells evoked by the mass activation of SOM neurons (right).

Figure 6

Ratio of the total charge of pIPSC (pIPSQ) to that of uIPSC (uIPSQ) in slice preprations. Each symbol represents the ratio value for the PV → Pyr cell connection (left column) and of SOM → Pyr cell connection (right column) obtained in the in vitro conditions. Filled symbol with vertical bar in the right represents means ± SEM. In the left column the value of SEM was very small so that it did not appear outside the symbol. Double asterisks indicate that the difference in the mean between the left and right columns is statistically significant at p < 0.01 (unpaired t-test).

Figure 7

Suppression of single PV neurons enhances visual responses of postsynaptic Pyr cells. (a) Membrane potentials of a Pyr cell (top row) and a presynaptic PV neuron (second row) to moving grating stimuli as shown at top. Stimuli were given in the period indicated by hatched area. Membrane potentials of the Pyr cell and the PV neuron during suppression of the latter neuron are shown in the third and fourth rows. (b) Raster plot of action potentials of the Pyr cell before (black dots) and during (red dots) suppression of the PV neuron. (c) Orientation tuning curves of the visual responses of the Pyr cell before (black) and during (red) the suppression of the PV neuron. Vertical bars indicate mean ± SEM of number of action potentials (APs) in 15 responses at each orientation.

Figure 8

Single PV neurons modify visual responses of postsynaptic Pyr cells while single SOM neurons mostly do not. (a) Membrane potentials of a Pyr cell (top row) and a presynaptic interneuron of the FS type (second row) to moving grating stimuli as shown at top. Stimuli were given in the period indicated by hatched area. Membrane potentials of the Pyr cell and the FS cell during activation of the FS cell are shown in the third and fourth rows. In the fourth row injection of depolarizing current induced repetitive generation of action potentials which are truncated. Scales at the right end of the bottom row apply all rows. (b) Single action potentials of the FS cell and evoked uIPSCs of the Pyr cell are superimposed. Scales of 20 pA and 20 mV apply to synaptic currents of the Pyr cell and membrane potentials of the FS cell, respectively. Arrangement of the two cells is shown in inset. Scale at bottom, 20 μm. (c) Changes in visual responses of postsynaptic Pyr cells by suppression (shown in the left side) and activation (right side) of presynaptic interneurons of the indicated types. Ratios to the control values were calculated by the number of action potentials or the mean value of depolarization during visual stimulation. The statistical analysis with t-test or Mann–Whitney U test was made for each pair by comparing the values obtained during suppression or activation of presynaptic interneurons with those before the presynaptic manipulation. Cells that showed the statistically significant change are indicated by shaded symbols. The mean for the PV/FS → Pyr cell pairs was 1.8 ± 0.4 in the suppression experiment and 0.4 ± 0.01 for the activation experiment. The mean for the SOM → Pyr cell pairs was 0.8 ± 0.1 in the suppression experiment and 0.9 ± 0.1 for the activation experiment. These data were obtained from 4 PV-ChR2-YFP, 3 VGAT-ChR2-YFP and 11 SOM-ChR2-YFP mice.