A corticothalamic switch: controlling the thalamus with dynamic synapses

Abstract

Corticothalamic neurons provide massive input to the thalamus. This top-down projection may allow the cortex to regulate sensory processing by modulating the excitability of thalamic cells. Layer 6 corticothalamic neurons monosynaptically excite thalamocortical cells, but also indirectly inhibit them by driving inhibitory cells of the thalamic reticular nucleus. Whether corticothalamic activity generally suppresses or excites the thalamus remains unclear. Here we show that the corticothalamic influence is dynamic, with the excitatory-inhibitory balance shifting in an activity-dependent fashion. During low-frequency activity, corticothalamic effects are mainly suppressive, whereas higher-frequency activity (even a short bout of gamma frequency oscillations) converts the corticothalamic influence to enhancement. The mechanism of this switching depends on distinct forms of short-term synaptic plasticity across multiple corticothalamic circuit components. Our results reveal an activity-dependent mechanism by which corticothalamic neurons can bidirectionally switch the excitability and sensory throughput of the thalamus, possibly to meet changing behavioral demands.

Figure 1. Low frequency CT stimuli modulate VPm excitability

(A) Schematic of the principal pathway for whisker-related sensory processing in rodent forebrain. (B) Image of a live TC slice (300 μm) from an Ntsr1-Cre mouse injected in barrel cortex with an AAV driving Cre-dependent ChR2-EYFP expression in L6 CT cells; Overlay of EYFP (green) and bright-field (gray). See also Figure S1. (C) Schematic of recording configuration: Optical stimulation of ChR2-expressing CT axons (green) and whole-cell recording (K+ solution) from a ChR2-negative VPm cell (black). (D) Top, control activity from a VPm neuron during injection of depolarizing current (overlay of 10 sweeps); middle, same cell and conditions except addition of CT stimulation (arrow, 1 ms flash; trials repeated at 0.1 Hz); bottom, raster plot of cell’s spiking. Control (black) and +CT (blue) trials are plotted as clustered groups for displays, but were interleaved during testing. (E) Population peri-stimulus time histogram (PSTH: 5 ms bins) plotting difference in spike rates for CT versus control trials (n = 9 cells from 5 mice). (F) Population PSTH showing the first 225 ms after CT stimulation. For (D–F), cells were injected with 30–110 pA to produce ~12 Hz baseline firing rates. CT stimulus intensities were 2x threshold for IPSPs (mean = 3.02 mW). See also Figure S2.

Figure 2. Trains of optical CT stimuli initially suppress and later enhance VPm excitability

(A) Schematic of recording configuration. (B) Same conditions as Figure 1D except repetitive CT activation (10 Hz - arrows); notice transition of CT effect from suppression to enhancement of spiking across the train. (C) Population PSTH (5 ms bins) plotting difference in spike rates for CT versus control trials (n = 12 cells from 6 mice). (D) Population PSTH showing the initial 100 ms of response after the first and tenth CT stimuli. Data are represented as mean ± SEM. (E) Fraction of bins in which the spike rates were suppressed (red) or enhanced (green) by CT stimulation versus stimulus number (bin counts taken from the population plot shown in (C)).

Figure 3. Synaptic excitation increases as inhibition decreases in VPm cells during repetitive CT activation

(A) Schematic of recording configuration. (B) IPSCs and EPSCs evoked in a VPm cell by 10 Hz optical activation of CT axons (average of 20 trials). EPSCs and IPSCs recorded at reversal potentials for inhibition (−74 mV) and excitation (+3 mV), respectively (Cs+ internal solution). (C) Average excitatory (G_e – green) and inhibitory (G_i – red) components of the total synaptic conductance (G_syn – black) evoked by the first and tenth CT stimuli (same cell as (B)). Insets show initial 10 ms of the responses. (D) Population data showing short-term dynamics of CT-EPSGs and di-synaptic IPSGs evoked in VPm cells (n = 8 cells). (E) Top: Mixed synaptic current recorded in a VPm cell voltage-clamped near spike threshold (−36 mV). Bottom: Calculated current, predicted for that same holding potential (−36 mV), based on linear extrapolation from the pure EPSGs and IPSGs plotted in panel B. (F) Population data showing the net charge resulting from the recorded and calculated/predicted current for each stimulus number (100 ms post-stimulus; n = 7 cells). Data are represented as mean ± SEM. See also Figure S3 and S4.

Figure 4. Role of fast glutamate receptors in CT modulation of VPm excitability

(A) Schematic of recording configuration. Inhibitory transmission was blocked by applying GABA_A and GABA_B receptor antagonists (picrotoxin: 50 μM; CGP: 2 μM). (B) CT-evoked EPSCs from a VPm cell in Control, DNQX and DNQX +APV conditions at the indicated membrane potentials (DNQX: 20 μM; APV: 50 μM). (C) Population plot showing the integrated current-voltage relationship for the CT-evoked EPSC (Control) as well as the isolated NMDAR and AMPAR components of the response (n = 6 cells from 5 mice). NMDAR-mediated responses were isolated by applying DNQX (n = 2) and measuring the response that remained, or by applying APV (n = 4) and subtracting the response that remained from the control response. AMPAR-mediated responses were isolated similarly (n = 4 APV cells; n = 2 DNQX cells). (D) Isolated NMDA-mediated synaptic current in response to repetitive 10 Hz optical activation of CT axons (DNQX present in the bath). (E) Schematic of recording configuration. Dynamic-clamp was used to compute a real-time current (I) output based on the recorded membrane potential (Vm) and three modeled CT-evoked conductances (g-AMPA, g-NMDA, g-GABA). Standard conductance waveforms were based on previously recorded data evoked by optical CT stimulation (Figures 3B and 4B–D). (F) Same type of excitability experiment as described in Figure 2. Left, overlay of 10 sweeps from a VPm cell with the three standard CT conductances applied. Trials with control spiking activity (i.e. no CT conductances) are not shown, but were interleaved during testing. Right, population PSTHs (5 ms bins) plotting difference in spike rates for the trials with CT conductances versus control trials (n = 14 cells from 6 mice). The two plots show the initial 100 ms of response after the first and tenth CT stimuli. (G) Similar excitability experiment, but the g-NMDA was removed (n = 11 cells from 6 mice). See also Figure S5 and S6. (H) Similar excitability experiment, but the g-AMPA was removed (n = 10 cells from 6 mice). Data are represented as mean ± SEM.

Figure 5. Synaptic dynamics can account for frequency-dependent switching in CT-triggered modulation of VPm excitability

(A) Same recording configuration as described in Figure 4E. (B) Different example cell but the same excitability experiment as described in Figure 4F. Left, overlay of 10 sweeps in which dynamic-clamp was used to model the three standard CT-evoked conductances. Right, population PSTHs (5 ms bins) plotting difference in spike rates (i.e., trials with CT conductances versus control trials) for the first and tenth CT stimuli (n = 14 cells from 6 mice). (C) Similar excitability experiment for the cell shown in B, but short-term facilitation of the excitatory conductances (g-AMPA and g-NMDA) was removed by statically repeating the first conductances across the train (n = 6 cells from 4 mice). (D) Similar excitability experiment for the cell shown in B, but short-term depression of the inhibitory conductance (g-GABA) was removed, again by repeating the first conductance across the train (n = 7 cells from 4 mice). Data are represented as mean ± SEM.

Figure 6. Mechanisms of dynamic reduction of CT-mediated inhibition in VPm cells

(A) Schematic of recording configuration. (B) IPSCs and EPSCs from a TRN cell in response to repetitive optical activation of CT axons (average of 20 trials; Cs⁺ internal solution). (C) Population plot showing the average synaptic charges for each stimulus (n = 5 cells from 2 mice). (D) TRN spike responses (cell-attached) to repetitive optical CT stimulation. For display, the trace was high passed filtered (100 Hz). Right, responses to the first and tenth stimuli. (E) Population plot showing the number of action potentials discharged by TRN cells versus stimulus number (n = 6 cells from 2 mice). See also Figure S7. (F) Electrical stimulation of cut TRN axons and whole-cell recordings from VPm cells; schematic of recording configuration. Glutamate receptors were blocked (20 μM DNQX, 50 μM APV). IPSCs measured at −34 or −39 mV. (G) Population IPSC dynamics from VPm cells (n = 13 cells from 4 mice) in response to simple electrical train stimulation of TRN axons across a range of fixed frequencies. (H) Left, IPSCs from a representative VPm cell in response to “TRN-burst” electrical stimulation. This pattern of stimulation matched the average burst pattern previously recorded from TRN cells in response to optical CT activation (during attached recordings as in D–E) (average of 20 sweeps). Right, overlay of the responses to the first and tenth stimulus. Electrical artifacts removed. (I) Mean IPSC dynamics of 6 cells (from 2 mice) electrically stimulated with the TRN-burst pattern and a control burst pattern, which consisted of 9 synaptic pulses at 350 Hz, repeated at 10 Hz. Also shown are IPSC data collected from the optical CT stimulation condition (Blue; from Figure 3). Data are represented as mean ± SEM.

Figure 7. Sustained optogenetic activation of CT cells in L6 generates local network gamma rhythms

(A) Schematic of recording configuration. (B) Left, Simultaneous local field potential (LFP) and intracellular recording of a ChR2-expressing CT cell in L6 during light ramp stimulation (1 s). LFP electrode located in L5b, at the level of CT cell apical dendrites. Right, Expanded traces of the period indicated by dashed red rectangle. Asterisk indicates truncated spike. (C) Average LFP power spectrum for same recording session shown in (B) (7 sweeps). For display, data were high-pass filtered (2 Hz). (D) Spike-triggered LFP averages for the LFP-cell pair in (B) (114 spikes). Similar spike-LFP phase-locking occurred in 6/6 cells (from 3 mice). (E) Left, Simultaneous LFP and intracellular voltage-clamp recording (from –54 mV) of a ChR2-expressing CT cell during light ramp stimulation (1 s). Right, Expanded traces of the period indicated by the dashed red rectangle. (F) Average IPSC power spectrum for the LED condition for the same recording session shown in (E) (7 sweeps). For display, data were high-pass filtered (2 Hz). (G) Cross-correlation of IPSCs and LFP during light ramp stimulation for the same LFP-cell pair shown in (E). IPSCs recorded in L6 CT cells were strongly phase-locked with the locally recorded LFP (Peak negative correlation: −0.67 ± 0.05, n = 5 cells from 3 mice). Data are represented as mean ± SEM.

Figure 8. Gamma-band activity in the CT L6 network initially suppresses and then enhances VPm excitability

(A) Schematic of recording configuration. (B) Left, Simultaneous intracellular recording of a ChR2-expressing CT cell and ChR2-negative VPm cell during light ramp stimulation over L6. The pair was not monosynaptically connected. Right, Expanded traces of the period indicated by the dashed red rectangle. Asterisks indicate truncated spikes. (C) Spike-triggered potential averages for the CT-VPm pair shown in (B) (47 spikes). (D) Left, Simultaneous LFP from cortex and intracellular voltage-clamp recording (–89 mV) from a ChR2-negative VPm cell during light ramp stimulation over L6 (1 s). Right, Expanded traces of the period indicated by the dashed red rectangle. (E) Average power spectrum of synaptic currents during LED stimulation for the VPm recording session shown in (D) (6 sweeps). For display, data were high-pass filtered (2 Hz). (F) Cross-correlation of VPm EPSCs and cortical LFP during light ramp stimulation for the LFP-cell pair shown in (D). EPSCs recorded in VPm cells were strongly correlated with the L6 LFP (Peak correlation: 0.59 ± 0.12, n = 2 cells from 2 mice). (G) Top, overlay of 10 sweeps from a VPm cell during injection of depolarizing current; bottom, same cell with same current injection during a 1 s light ramp over L6. (H) Mixed synaptic currents (V_m −54 mV) from cell shown in (G). (I) Average synaptic current (blue trace; SEM shown in gray) evoked by optical ramp stimulation for the 7 cells tested in voltage-clamp (V_hold ~−54 mV). Net currents were outward during the first half of the optical stimulus and inward during the second half. Data are represented as mean ± SEM. See also Figure S8.