Network mechanisms of theta related neuronal activity in hippocampal CA1 pyramidal neurons

Abstract

Although hippocampal theta oscillations represent a prime example of temporal coding in the mammalian brain, little is known about the specific biophysical mechanisms. Intracellular recordings support a particular abstract oscillatory interference model of hippocampal theta activity, the soma-dendrite interference model. To gain insight into the cellular and circuit level mechanisms of theta activity, we implemented a similar form of interference using the actual hippocampal network in mice in vitro. We found that pairing increasing levels of phasic dendritic excitation with phasic stimulation of perisomatic projecting inhibitory interneurons induced a somatic polarization and action potential timing profile that reproduced most common features. Alterations in the temporal profile of inhibition were required to fully capture all features. These data suggest that theta-related place cell activity is generated through an interaction between a phasic dendritic excitation and a phasic perisomatic shunting inhibition delivered by interneurons, a subset of which undergo activity-dependent presynaptic modulation.

Figure 1

Experimental implementation of subtractive and divisive forms of inhibition. (a) Schematic of the experimental configuration. A dendritic whole-cell electrode was used to inject a depolarizing sine wave current while the corresponding somatic and dendritic voltage responses were recorded in the absence (i) or presence (ii) of a somatic 180° out-of phase hyperpolarizing sine wave current injection, or (iii) ChR2-photostimulation. (b) Two-photon image stack of a CA1 pyramidal neuron (red: Alexa 594) in a dense network of ChR2-GFP+ fibers (green) concentrated in the pyramidal layer in a hippocampal slice from a PV-Cre mouse injected with rAAV-FLEX-rev-ChR2-GFP. Blue dots: ChR2-photostimulation grid. (c) Individual (left, 1-25 locations, 100 ms interval) and synchronous (right, 1-25 locations, 3 ms interval) inhibitory postsynaptic currents (VC: voltage-clamp) potentials (CC: current-clamp) evoked by photostimuli corresponding to the locations indicated by letters on b (a: 1^st, 13^rd, 25^th; b: 2^nd 14^th; c: 3^rd 15^th;d: 4^th, 16^th;e: 5^th, 17^th; f: 6^th 18^th; g: 7^th, 19^th; h: 8^th, 20^th; i: 9^th, 21^st; j: 10^th 22^nd; k: 11^th, 23^rd; l: 12^th,24^th points) and recorded at three different membrane potentials. Blue tickmarks: timing and relative intensity of laser pulses (d) Dendritic (upper, Dend. V_m) and somatic (lower, Soma V_m) membrane potential responses obtained for dendritic current injection of increasing amplitude alone (left), plus in the presence of somatic hyperpolarizing current injection (middle) or with ChR2-photostimulation (right). (e) Subthreshold input-output curves recorded at the dendrite (upper) and at the soma (lower) for the three different input conditions.

Figure 2

Soma-dendrite interference induces phase precession. (a) Dendritic excitation plus somatic current injection. Concatenation of dendritic current injection (upper gray, 0, 40, 75, 130, 200, 300, 500, 670, 1300 pA), soma membrane potential (soma V_m, dark blue), 3-8 Hz bandpass-filtered V_m (black) and soma current injection (light blue) waveforms. Hyperpolarizing 5 Hz sine wave-shaped somatic current injections (100 pA constant amplitude) are paired with depolarizing 5 Hz sine wave-shaped current injections into the distal apical dendrites. As the dendritic current amplitude is increased the spike latency (upper) and timing of the subthreshold peak depolarization (lower) advanced (grey dots: depolarization peak, black tickmarks: spike peak). (b) Dendritic excitation plus perisomatic conductance stimulation. Concatenation of dendritic current injection (upper gray, 0, 40, 75, 130, 200, 300, 500, 670, 1300 pA), soma V_m (dark green), bandpass-filtered V_m (black) and approximate perisomatic conductance (light green) waveforms. Increasing dendritic depolarizing current injection is paired with 5 Hz sine wave-shaped ChR2-photostimulation. Spike latency (upper) and timing of the subthreshold peak depolarization (lower) advances with increased dendritic current (grey dots: depolarization peak, black tickmarks: spike peak). (c) Dendritic excitation only. Concatenation of dendritic current injection (upper gray, 40, 75, 130, 200, 250, 300, 500, 670 pA), soma V_m (dark red), bandpass-filtered V_m (black) and missing soma inhibitory (light red dashed) waveforms. Increasing dendritic current injection is delivered alone without somatic inhibition. Spike latency (upper) and timing of the subthreshold peak depolarization (lower) advance only slightly with increased dendritic current (grey dots: depolarization peak, tickmarks: spike peak).

Figure 3

Grouped data showing phase advance and subthreshold potential profiles. Action potential phase (open symbols) and mean subthreshold potential peak time (solid symbols, mean ± s.e.m, n = 6 cells) plotted versus dendritic current injection with reference to the dendritic or somatic input cycle (circles) or the peak of the intracellularly recorded subthreshold potential (triangles) for somatic inhibition delivered as somatic hyperpolarizing current injections (a, blue, n = 6 cells), synaptic conductance changes (b, green, n = 6 cells) or no inhibition (c, red, n = 6 cells). Spike times are fit by single exponential functions. d, Amplitude of somatic subthreshold potentials versus dendritic current injection (normalized to maximum) for somatic inhibition delivered as hyperpolarizing current injections (blue, n = 6), perisomatic conductance changes (green, n = 6) or no inhibition (red, n = 6).

Figure 4

Delayed inhibition dissociates place cell firing rate and phase. (a) Left, experimental configuration: somatic filtered sine wave current injection paired with temporally-focused two-photon ChR2-photostimulation (2pTeFo) of inhibitory profiles. Right, two-photon stack of a CA1 pyramidal neuron (red) and ChR2-GFP+ profiles of PV-expressing interneurons. Grey dots: photostimulation grid. Individual inhibitory postsynaptic currents (VC: voltage-clamp) evoked by random pattern photostimulation (bottom right). Red tickmarks: 880 nm laser pulse timing. (b) Stimulation sequence arranged to evoke a bimodal (upper left, green) or an increasing (lower left, blue) amplitude sequence that generated a symmetrical (upper right; g_inh.(S), grey: individual traces, green: average) or a delayed (lower right; g_inh.(D); grey: individual traces, blue: average) inhibition profiles with synchronous photostimulation (3 ms interval), respectively. (c) Repeated 2pTeFo photostimulation: 1^st-9^th theta cycles with ramp-up depolarizing current injections (i_depol., dark grey) and symmetrical inhibition (V_m, soma g_inh. only, green); 10^th-18^th theta cycles with ramp-down current injections and delayed inhibition (V_m, soma g_inh. only, blue). V_m, soma: soma membrane potential, filtered: 3-8 Hz bandpass-filtered V_m, external reference: population inhibitory waveform. Black tickmarks: spike timing, grey dots: subthreshold peak. (d) Summary of mean spike phase (green and blue circles) and the mean phase of peak subthreshold potential (grey circles, mean ± s.e.m, n = 9) for increased and decreased levels of depolarization paired with symmetrical or delayed inhibition, respectively (n = 9). Solid lines: exponential fits. (e) Summary of mean number of spikes per cycle versus levels of current injections (mean ± s.e.m, n = 9). Solid lines: linear fits.

Figure 5

Presynaptic cannabinoid receptor activations delays the time to peak of inhibition. (a) Type I cannabinoid receptor (CB1R) agonist-sensitive inhibitory postsynaptic currents evoked by repeated 2pTeFo ChR2-photostimulation at a single location in the pyramidal layer in a GAD65-Cre mice in the presence of CB1R agonist (1 μM WIN55,212-2, blue) and following CB1R antagonist (10 μM AM251, green) application. Red tickmarks indicate the timing of the 2pTeFo photostimulation pulses at 880 nm. (b) Representative membrane potential traces of CB1R agonist-sensitive inhibitory responses evoked by burst 2pTeFo photostimulation (36 stimuli; #1 - #6 non-overlapping locations repeated six times with a 1 ms duration and a 3 ms interval, ∼40 Hz cycle) alone (lower traces) and paired with depolarizing current injections (+200 pA, upper traces), in the presence of WIN55,212-2 (1 μM, blue) and following subsequent application of AM251 (10 μM, green). Note that the timing of APs is shifted forward in the presence of CB1R agonist. Red tickmarks indicate the timing and relative intensity of the 2pTeFo photostimulation pulses at 880 nm. c, Summary data of peak phase of inhibitory responses evoked by 2pTeFo photostimulation in PV-Cre mice (open circles, n = 2) and peak phase of CB1R agonist-sensitive inhibitory responses evoked by 2pTeFo photostimulation in GAD65-Cre mice (right, solid gray circles, n = 5, black: mean ± s.e.m), in the presence of WIN55,212-2 (1 μM) and following subsequent application of AM251 (10 μM).