Control of timing, rate and bursts of hippocampal place cells by dendritic and somatic inhibition

Abstract

A consortium of inhibitory neurons control the firing patterns of pyramidal cells, but their specific roles in the behaving animal are largely unknown. We performed simultaneous physiological recordings and optogenetic silencing of either perisomatic (parvalbumin (PV) expressing) or dendrite-targeting (somatostatin (SOM) expressing) interneurons in hippocampal area CA1 of head-fixed mice actively moving a treadmill belt rich with visual-tactile stimuli. Silencing of either PV or SOM interneurons increased the firing rates of pyramidal cells selectively in their place fields, with PV and SOM interneurons having their largest effect during the rising and decaying parts of the place field, respectively. SOM interneuron silencing powerfully increased burst firing without altering the theta phase of spikes. In contrast, PV interneuron silencing had no effect on burst firing, but instead shifted the spikes' theta phase toward the trough of theta. These findings indicate that perisomatic and dendritic inhibition have distinct roles in controlling the rate, burst and timing of hippocampal pyramidal cells.

Figure 1

Self movement–controlled local stimuli generate hippocampal cell sequences. (a) Experimental setup for optical probe recording in head-fixed mice during treadmill running. Visual and tactile stimuli of the belt are illustrated. Three reflector cubes placed at equal intervals (blue, red and orange colors are only for illustration purposes) triggered photo-beam sensors for the control of water reward delivery and laser stimulation. (b) Running speed as a function of belt position during the first and last days of training in a mouse. Each line is a single trial. Sucrose-water reward was delivered on every trial at position 0. (c) Average speed between the 57- and 113-cm positions and number of trials completed after 30 min (mean ± s.d. of three mice). (d) Wide-band (1 Hz to 5 kHz) recording of electrical activity on a single shank of the optical probe. Color ticks indicate spikes of isolated CA1 pyramidal cells. (e) Place field of a single neuron. Mean firing rate, single trials and spike theta phase as a function of belt position. (f) Activity of 55 place cells recorded simultaneously. Each row is one neuron normalized firing rate. Successive trials are concatenated (only trials 90–130 are shown) and the cells are ordered according to their place field position. Right, average of 131 trials.

Figure 2

Light-assisted silencing of PV and SOM neurons. (a) Specific halorhodopsin expression in hippocampal interneurons. Top, low-magnification images of halorhodopsin-GFP in PV and SOM interneurons following viral injection. Bottom, higher magnification of the boxed areas. Ori, stratum oriens; pyr, pyramidal layer; rad, stratum radiatum; lm, stratum lacunosummoleculare. (b) Peristimulus histograms of simultaneously monitored neuronal responses to 1-s-long light pulses (50 pulses, 7-s interpulse interval), aligned to their respective recording shanks in a PV-Cre mouse. (c) Higher magnification of the boxed peristimulus histograms in b. (d) Percent decrease and increase of firing rates of PV cells (black, 69 cells) and other non-PV neurons (white, 722 cells), respectively, shown separately for each shank (mean ± s.d. of all neurons recorded from 11 sessions in 5 mice). Note that the largest effect occurred on the shanks carrying optical fibers. On shanks located further away from the stimulated volume, the magnitude of changes was decreased and the number of detectable PV neurons was reduced (n = 1 and 0 on the seventh and eighth shanks, respectively). (e) Normalized activity of simultaneously recorded PV interneurons and pyramidal cells during alternate light and light-off (control) trials. (f) Example PV interneuron and pyramidal cell firing patterns. Each line is a single trial. The color scale is the same as that used in e. Right, session average firing rates of the two cells (averages of the 50 trials). The blue line indicates control trials and the red line indicates light trials. Magenta dots indicate significant rate differences in different pixels (P < 0.05, rank test, 1,000 shuffles, see Online Methods).

Figure 3

Firing activity of different cell types during SWRs and theta oscillations. (a) Left, population average of peri-ripple histograms for PV (n = 23), SOM (n = 8), PV like (n = 89), non-bursting (n = 80) and pyramidal cells (n = 707). Right, distribution of ripple modulation index in the five cell groups. Higher positive indexes indicate larger increases in firing rate during ripples, whereas larger negative indexes indicate larger decreases in firing rate during ripples. The bimodal behavior of SOM neurons indicates that they represent a mixture of oriens-lacunosum moleculare (ripple activated) and bistratified (ripple suppressed) interneurons. (b) Left, population average of theta phase histograms for PV (n = 30 cells, 30,369 spikes), SOM (n = 9 cells, 7,078 spikes), PV like (n = 98 cells, 116,616 spikes), non-bursting (n = 56 cells, 16,306 spikes) and pyramidal cells (n = 356 cells, 38,623 spikes) during control trials. Right, distribution of mean theta phases.

Figure 4

Light-induced changes in firing patterns. (a) Normalized activity of PV interneurons, non-PV interneurons (PV-like interneurons and non-bursting cells) and pyramidal cells during control (left) and light (right) trials in PV-Cre mice (Supplementary Figs. 5–8). Each line is the average of control or light trials for a single neuron. The cells are ordered according to the position of their place field. Vertical yellow lines delimit the light-stimulated segment of the belt. White lines indicate group average. (b) Population averages of firing rates for each neuron group during control (blue) and light (red) trials in PV-Cre mice. Numbers in each panel indicate the number of cells included in each average. Pyramidal neurons whose place fields completely covered the stimulated area (in) or that had no overlap with the stimulated area (out) are shown separately. Magenta dots indicate significant rate differences in different pixels (P < 0.05, two-tail paired t test). (c) Data are presented as in b for neurons in SOM-Cre mice. (d) Top, binning of the place fields. Bottom, percent rate increase in each bin (mean ± s.e.m.) in PV-Cre and SOM-Cre mice. The percent rate increase was first computed for individual neuron in each place field bin overlapping with the stimulated area of the belt and then the average increase of the neuron population was computed for each bin (*P < 0.05, **P < 0.005, ^#P < 0.0005, unpaired t test).

Figure 5

SOM interneurons control spike bursts in pyramidal cells. (a) LFP and unit activity of one pyramidal cell example, illustrating single spikes and burst discharge. (b) Average (mean ± s.e.m.) probability distribution of interspike intervals of pyramidal neurons. Control (blue) and light (red) histograms include 20,514 and 23,557 spikes for PV-Cre mice (123 cells) and 16,336 and 19,680 spikes for SOM-Cre mice (98 cells) (comparison between control and light, *P < 0.05, ^#P < 0.0005; two-tail paired t test). (c) Relative increase (mean ± s.e.m.) of occurrence for different burst lengths (comparison between PV-Cre and SOM-Cre mice, *P < 0.05, ^#P < 0.0005; unpaired t test).

Figure 6

Theta oscillations are not affected by focal disinhibition. (a) Wide-band LFP segment during light stimulation (green line) and theta band–filtered trace (bottom). Right, three theta cycles during lights off and light on conditions. (b) Power spectra calculated separately for control and light trials of a representative recording session in a PV-Cre mouse. Inset, zoom on theta band. (c) Average power spectra of all sessions from PV-Cre mice. Dash lines indicate standard error (n = 8 sessions, P > 0.05, two-tail paired t test).

Figure 7

Within-theta timing of spikes is regulated by PV interneurons. (a) Left, place field of an example pyramidal cell from a PV-Cre mouse and spike theta phase as a function of belt position during control (blue) and light (red) trials. Right, theta phase histogram of spikes in bin 4 of the place field (control, n = 60 spikes, 18 trials; light, n = 69 spikes, 18 trials). The arrow indicates the direction of phase shift during PV disinhibition. (b) Comparison of rate and theta phase shift of spikes in PV-Cre and SOM-Cre mice, shown separately for rate-affected (upward arrow) and non-affected subgroups (O). Top, subgroup mean firing rates during control and light trials (PV, 78 non-affected cells, 69 rate-affected cells; SOM, 59 non-affected cells, 51 rate-affected cells). Magenta dots indicate significant rate differences in different pixels (P < 0.05, two-tail paired t test). Bottom, subgroup mean phase shifts (±s.e.m.). P values for bins with significant phase shifts are indicated (see Online Methods). (c) Mean theta phases (± s.e.m.) during control and light trials for PV (n = 30), SOM (n = 9), PV-like (n = 12 and 86 in PV-Cre and SOM-Cre mice respectively), non-bursting (n = 23 and 33 in PV-Cre and SOM-Cre mice, respectively) and pyramidal cells (from PV-Cre mice, same cell numbers as in b). Orange shading indicates data from SOM-Cre mice. Pyramidal neurons in PV-Cre mice with no rate change to light (O) and with significant increases (upward arrow) to light are shown separately. Note the late-to-early phase advancement of spikes across the bins of the field and the significant phase shifts in the rate affected group. Right, average theta phase histograms for PV and SOM interneurons and in bins of place field for the rate affected pyramidal cells (control, n = 5,872 spikes; light, 8,276 spikes; see also Supplementary Fig. 9).