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4-Hz oscillations synchronize prefrontal–amygdala circuits during fear behavior

Abstract

Fear expression relies on the coordinated activity of prefrontal and amygdala circuits, yet the mechanisms allowing long-range network synchronization during fear remain unknown. Using a combination of extracellular recordings, pharmacological and optogenetic manipulations, we found that freezing, a behavioral expression of fear, temporally coincided with the development of sustained, internally generated 4-Hz oscillations in prefrontal–amygdala circuits. 4-Hz oscillations predict freezing onset and offset and synchronize prefrontal–amygdala circuits. Optogenetic induction of prefrontal 4-Hz oscillations coordinates prefrontal–amygdala activity and elicits fear behavior. These results unravel a sustained oscillatory mechanism mediating prefrontal–amygdala coupling during fear behavior.

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Figure 1: Freezing behavior is triggered by internally generated mechanisms.
Figure 2: Emergence of dmPFC 4-Hz oscillations during freezing behavior.
Figure 3: dmPFC 4 Hz oscillations predict freezing.
Figure 4: BLA 4-Hz oscillations emerge during freezing.
Figure 5: Synchronization of dmPFC and BLA 4-Hz oscillations during freezing.
Figure 6: 4-Hz oscillations synchronize dmPFC–BLA spiking activity.
Figure 7: Optogenetic induction of dmPFC 4-Hz oscillations drives freezing.

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Acknowledgements

We thank the members of the Herry laboratory for discussions and comments on the manuscript and K. Deisseroth (Stanford University) and E. Boyden (Massachusetts Institute of Technology) for generously sharing material. This work was supported by grants from the French National Research Agency (ANR-2010-BLAN-1442-01; ANR-10-EQPX-08 OPTOPATH; LABEX BRAIN ANR 10-LABX-43, LABEX TRAIL ANR 10-LABX-57), the European Research Council (ERC) under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC grant agreement no. 281168, the Conseil Regional d'Aquitaine (C.H.), the Fondation pour la Recherche Médicale (FRM) (F.C.), the CNRS ATIP program (2014) and the city of Paris (Grant Emergence 2014), the French National Research Agency (ANR-10-LABX-54 MEMO LIFE; ANR-11-IDEX-0001-02 PSL) (K.B.), Munich Cluster for Systems Neurology (SyNergy, EXC 1010), Deutsche Forschungsgemeinschaft Priority Program 1665 and 1392 and Bundesministerium für Bildung und Forschung via grant no. 01GQ0440 (Bernstein Centre for Computational Neuroscience Munich) (A.S.) and a scholarship from the Erasmus Mundus program Neurasmus (N.K.).

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Authors and Affiliations

Authors

Contributions

S.B., K.B., F.C., J.C., C.D., N.K., S.K., R.R.R., A.S. and H.W. performed the experiments and analyzed the data. J.C., C.D., N.K. and C.H. designed the experiments. C.H. wrote the paper.

Corresponding authors

Correspondence to Nikolaos Karalis or Cyril Herry.

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The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Emergence of dmPFC and BLA 4-Hz oscillations during freezing behavior.

(a) Representative power spectrum of dmPFC LFPs recorded during Retrieval for freezing and no freezing periods. (b) Representative signal to noise ratio (SNR) of dmPFC 4 Hz oscillation (2-6 Hz) during Habituation (Hab.) and Retrieval (Ret.). (c) Representative power spectrum of BLA LFPs recorded during Retrieval for freezing and no freezing periods. (d) Representative signal to noise ratio (SNR) of BLA 4 Hz oscillation (2-6 Hz) during Habituation (Hab.) and Retrieval (Ret.). Similar results were obtained for the 13 animals used in these experiments.

Supplementary Figure 2 Development of dmPFC and BLA 4-Hz oscillations during fear conditioning.

(a) Representative spectrogram of dmPFC LFPs recorded during auditory fear conditioning for successive CS+-US associations (top red lines: CS+ duration). White lines on the spectrogram indicate immobility/freezing episodes. (b, c) Averaged dmPFC (b) and BLA (c) power during CS+ presentations throughout the conditioning session (n = 4 mice, one way repeated measures ANOVA, dmPFC F(3,4) = 3.617, * P = 0.0372; BLA: F(3,4) = 4.794, * P = 0.0153). a.u.: arbitrary units. Spectral power in log scale. Error bars: mean ± s.e.m. For the representative example in (a), similar results were obtained for the 4 animals used in these experiments.

Supplementary Figure 3 Emergence of dmPFC 4-Hz oscillations during contextual fear behaviour.

(a, b) Protocol and behavioural results. During Habituation (Hab.), mice (n = 6) exhibited low freezing whereas following contextual fear conditioning (Day 2: Retrieval), freezing values were significantly increased (Paired t-test, Hab. versus Retrieval: t(5) = -11.639, *** P < 0.001). (c) Representative dmPFC raw LFP traces recorded during the Retrieval session. 4 Hz oscillatory activity is visible during freezing behaviour. (d) Representative spectrogram of dmPFC LFPs at a fine time resolution during freezing episodes at Retrieval. White lines on the spectrogram indicate freezing episodes. (e) Averaged power spectrum of dmPFC LFPs recorded during Retrieval for freezing and no freezing periods (n = 6 mice). Inset: Averaged dmPFC 2-6 Hz power during Retrieval for freezing and no freezing periods (Paired t-test, Freezing versus No Freezing: t(5) = -6.039, *** P < 0.001). a.u.: arbitrary units. Spectral power in log scale. Shaded area and error bars: mean ± s.e.m. For the representative examples (c, d), similar results were obtained for the 6 animals used in these experiments.

Supplementary Figure 4 Relation of dmPFC and BLA 4-Hz oscillations with freezing, immobility and motor behavior.

(a) Representative spectrograms of BLA LFPs recorded during Habituation. White lines on the spectrogram indicate immobility periods. (b) Correlation coefficient between the power envelope of the 4 Hz oscillations (2–6 Hz bandpass) recorded in the BLA and immobility/freezing behaviour during Habituation (Hab.) and Retrieval sessions (n = 13 mice, paired t-test, BLA: t(12) = -4.871, *** P < 0.001). (c) Receiver operating characteristics (ROC) analyses performed on Naïve Bayes (NBC) and Support Vector Machine (SVM) classifiers trained on 4 Hz signal to noise ratio (SNR) of the BLA during freezing episodes (see Methods). Inset: averaged area under the curve for NBC and SVM classifiers compared to corresponding shuffled data. (d) Percentage of accuracy of the NBC and SVM classifiers to predict freezing behaviour based on BLA 4 Hz SNR (see Methods). (e) Correlation analysis performed between freezing and 4 Hz episodes for the BLA (n = 13 mice; BLA: Pearson's r coefficient = 0.01, P = 0.974). (f) Location of recording sites in the lateral (lPAG) and ventrolateral periaqueductal grey (vlPAG) (n = 5 mice). (g) Left, Averaged power spectrum of PAG LFPs recorded during Retrieval for freezing and no freezing periods (n = 5 mice). Right, Averaged PAG 2-6 Hz power during Retrieval for freezing and no freezing periods (n = 5 mice, paired t-test, t(4) = 0.098, P = 0.926). Shaded areas and error bars: mean ± s.e.m. Spectral power in log scale. a.u.: arbitrary units. For the representative example in (a), similar results were obtained for the 13 animals used in these experiments.

Supplementary Figure 5 Reversible inactivation of the medial septum does not block dmPFC 4-Hz oscillations.

(a, b) Experimental strategy and location of injections sites in the medial septum (MS) and composite transmission light and epifluorescence micrograph showing the MS targeted injection of muscimol covalently bound to a fluorescent tag (bottom, dipyrromethene boron difluoride (BODIPY), red). (c) Power spectra of dmPFC LFPs before (Day 2: Test 1), following (Day 3: Inact.) and after (Day 4: Test 2) MS inactivation (n = 6). (d) Top, Quantification of dmPFC LFP 4 Hz power (2-6 Hz) before, during, and after MS inactivation. (n = 6 mice, One-way repeated measure ANOVA: F(2,5) = 2.167, P = 0.165. Bottom, Quantification of dmPFC LFP theta power (6-12 Hz) before, during, and after MS inactivation. MS inactivation significantly reduced dmPFC theta power (n = 6 mice, One-way repeated measure ANOVA: F(2,5) = 19.245, P < 0.001, paired t-test Bonferroni corrected, Test 1 versus Inact.: t(5) = 6.155, *** P < 0.001; Inac. versus Test 2: t(5) = 4.173, ** P = 0.006). (e) Representative spectrograms of dmPFC LFPs before (Test 1), during (Inactivation) and after (Test 2) MS inactivation (red tick represents single CS+ pips). White lines on the spectrogram indicate immobility/freezing episodes. (f) Percentage of dmPFC neurons phase-locked to 4 Hz (top), or theta (bottom) oscillations (6-12 Hz). (Test 1: n = 80 neurons; Inact.: n = 100 neurons; Test 2: n = 120 neurons. 4 Hz: One-way repeated measure ANOVA: F(2,5) = 1.245, P = 0.329; theta: One-way repeated measure ANOVA: F(2,5) = 9.858, P = 0.004, Bonferroni corrected t-tests; Test1 versus Inactivation: t(5) = 2.956, P = 0.043; Inactivation versus Test 2: t(5) = 4.347, P = 0.004). a.u.: arbitrary units. Shaded areas and error bars: mean ± s.e.m. For the representative examples in (a, e), similar results were obtained for the 6 animals used in these experiments.

Supplementary Figure 6 dmPFC local theta resetting and 4-Hz oscillations are independent phenomena.

(a, b) Averaged raw (a) and Z-score normalized (b) LFP spectrogram in the 2-12 Hz frequency band around CS+ pip presentations during freezing episodes. CS+-associated theta was observed as a single burst occurring right after CS+ and spanning ~300 ms with frequency between 8 and 12 Hz (b). On the contrary 4 Hz oscillation was present throughout the triggered spectrogram, due to the co-occurring freezing behaviour and was not modulated by CS+ presentation (a). (c) Overlaid dmPFC local theta and 4 Hz phases of CS+-triggered LFP traces from a representative animal, illustrating theta but not 4 Hz phase resetting induced by CS+ pip presentations during freezing episodes. (d) LFP phase spectrogram in the 2-12 Hz frequency band around CS+ pip presentations from the same animal as in (c) revealing a sharp and stable versus a broad and unstable CS+-evoked phase for local theta and 4 Hz oscillations, respectively. (e) Color coded mean resultant length (MRL) of phases across animal for each time-frequency bin in the 2-12 Hz frequency band around CS+ pip presentations during freezing episodes (n = 13 mice). A low MRL value indicates an unstable phase (no modulation nor resetting of 4 Hz oscillations) whereas a high MRL value relates to a consistent phase (8-12 Hz theta burst and resetting). (f) Mean MRL for dmPFC 4 Hz (blue line) and 8- 12 Hz theta (red line) revealing the changes in phase stability triggered by CS+ pip presentations during freezing episodes (n = 13 mice). Inset: Averaged MRL after (0 to 300 ms) CS+ pip onset (n = 13 mice, 12 CS+, paired t-test, 4 Hz versus Theta: t(12) = 4.417, *** P < 0.001). Shaded area and error bars: mean ± s.e.m. Spectral power in log scale. For the representative examples in (c), similar results were obtained for the 13 animals used in these experiments.

Supplementary Figure 7 dmPFC and BLA directional analysis.

(a) Representative circular distribution of the phase differences between the filtered dmPFC and BLA LFP signals recorded for freezing episodes during Retrieval compared to a control bootstrap simulated phase distribution. (b) Representative dmPFC and BLA LFPs imaginary coherence (Im. coherence, see Methods) for Freezing and No Freezing episodes recorded during Retrieval (shaded areas: mean ± s.e.m.). (c) Representative Granger Causality (GC) analysis performed between dmPFC and BLA LFPs during freezing behaviour. (d) Granger Causality (GC) analyses performed between dmPFC and BLA LFPs during No Freezing periods at retrieval (n = 13 mice). No directional effect was observed in these conditions (b, c) GC analyses performed between dmPFC and BLA LFPs during Freezing (b) and No Freezing (c) periods at Retrieval and for which the phase or amplitude of 4 Hz LFPs was permuted (see Methods). The absence of directional effect in the freezing condition indicates that the direction of interaction between dmPFC and BLA is highly dependent on both 4 Hz phase and amplitude (n = 13 mice). For the representative examples in (a, b, and c), similar results were obtained for the 13 (a, b) and 12 (c) animals used in these experiments.

Supplementary Figure 8 Separation between dmPFC and BLA putative principal neurons and putative interneurons.

(a, b) Location of recording sites in the dmPFC (a) and BLA (b). (c) Among dmPFC recorded neurons, 27.56 % were classified as putative inhibitory neurons (INs, red circle) and 72.44 % as putative projection neurons (PNs, blue circle) using an unbiased unsupervised cluster separation algorithm based on three extracellular electrophysiological properties: firing frequency, spike half width (SHW) and spike area under waveform (AUC) peak. Inset, average waveform of a representative PN and IN illustrating the methodology used to quantify SHW and AUC. (d) Among BLA recorded neurons, 17.24 % were classified as putative inhibitory neurons (INs, red circle) and 82.76 % as putative projection neurons (PNs, blue circle) using the same methodology as in (c). (e, f) Cumulative distribution of firing rate ratio for freezing and no freezing periods of dmPFC (e) and BLA (f) putative PNs. Inset, percentage of phase-locked and non phase-locked dmPFC (e) and BLA (f) neurons to 4 Hz oscillations characterized by enhanced firing rate during fear behaviour (n = 13 mice, dmPFC: 46 cells; BLA: 21 cells).

Supplementary Figure 9 Oscillatory activity of dmPFC and BLA cells.

(a) Representative auto-correlogram from a dmPFC putative principal neuron on which a Gabor function was fitted to evaluate its oscillatory activity (see Methods section). (b, c) Percentage of dmPFC (b) and BLA (c) putative excitatory principal neurons and inhibitory interneurons that were phase-locked to 4 Hz and displaying 4 Hz oscillatory activity. (d) Left, Representative post-stimulus time histogram (PSTH) during optogenetic stimulation of an identified dmPFC parvalbumin-expressing (PV) interneuron. Blue shaded area corresponds to the light stimulation period. Right, Representative spiking activity of an identified PV interneuron in relation to dmPFC LFPs recorded during freezing behaviour. (e) Representative (blue bars) and averaged (red dots) phase-histogram of optogenetically identified dmPFC PV interneurons (n = 18 PV cells collected in 13 mice). (f) Percentage of optogenetically identified dmPFC PV interneurons phase locked to dmPFC 4 Hz oscillations (13 out of 18 PV cells (72%)). (g) Circular distribution of the 4 Hz oscillation preferred phase for populations of optogenetically identified dmPFC PV interneurons phase-locked to 4 Hz during freezing behaviour. (h) Representative auto-correlogram from optogenetically identified dmPFC PV interneuron on which a Gabor function was fitted to evaluate its oscillatory activity (see Methods section). (i) Corresponding representative correlation analysis performed between spike counts and Gabor function amplitude (Pearson's r coefficient = 0.79, P < 0.001). (j) Percentage of optogenetically identified dmPFC PV interneurons phase-locked to 4 Hz displaying 4 Hz oscillatory activity (11 out of 13 PV cells (84.6%)). For the representative traces in (a, d-e, and h-i), similar results were obtained for the animals used in these experiments (13 mice in a, and 13 mice in d-e and h-i).

Supplementary Figure 10 4-Hz analog stimulation–mediated freezing behavior.

(a) Percentage of freezing displayed by ChR2 mice in context A during (Day 1 Stim. Ctx A), or 24 hours after (Day 2 No Stim. Ctx A) 4 Hz optogenetic analog stimulation, or 48 hours later in a neutral context (Day 2 No Stim. Ctx B) (n = 8 mice, Mann-Whitney U test, Day 1 or Day 2 versus Day 3: U = 2 and 0, ** P = 0.002; *** P < 0.001). (b) Left, representative micrograph of the motor cortex (M1) in a PV-cre mouse infected with ChR2. Scale bar = 0.5 mm. Right, Percentage of freezing displayed by PV-Cre mice infected in the motor cortex with ChR2 during motor cortex 4 Hz analog stimulation (n = 4 mice; Paired t-test, t(3) = 0.623, P = 0.578). (c) Left, representative micrograph of BLA in a PV-cre mouse infected with ChR2. Scale bar = 0.4 mm. Right, Averaged peri-stimulus time histograms (PSTH) showing that light activation of BLA PV interneurons can suppress the activity of BLA putative principal neurons (n = 4 cells from 2 mice). (d) Percentage of freezing displayed by PV-Cre mice infected in the BLA with ChR2 during baseline conditions (n = 5 mice; Paired t-test, t(4)= -0.721, P = 0.511). (e, f) Percentage of dmPFC (e, left) and BLA (f, left) neurons significantly phase-locked to 4 Hz oscillations and circular distribution of the 4 Hz oscillation preferred phase for populations of dmPFC (e, right) and BLA (f, right) phase-locked putative excitatory principal neurons (PNs) and inhibitory interneurons (INs) during dmPFC 4 Hz analog optogenetic stimulation (dmPFC: 25 PNs, 9 INs; BLA: 3 PNs, 1 INs). (g) Left, behavioral protocol. Right, Percentage of freezing behaviour to CS-, and CS+ presentations during no-light or light exposure on Test 1 (n = 5 mice, Paired t-test, CS+ no light versus CS+ light, t(4)= -4.265, P = 0.013). Freezing levels to CS+ presentations during Test 2 normalized to CS+ evoked freezing levels during Test 1 (n = 5 mice, Paired t-test, CS+ Day 2 versus CS+ Day3, t(4)= -1.080, P = 0.341). Error bars: mean ± s.e.m. For the representative micrographs in (b and c), similar results were obtained for the animals used in these experiments (4 mice in b, and 5 mice in c). Data for panels (a) and (g) overlap with the data represented in Figure 7c and 7f, respectively.

Supplementary Figure 11 Circuit and potential mechanism of 4-Hz-mediated freezing behavior.

(a) During freezing behaviour, the preferential dmPFC to BLA coupling at 4 Hz allows the expression of conditioned fear behaviour through the recruitment of BLA circuits contacting the central amygdala (CE) which project to ventrolateral periaqueductal grey (vlPAG). (b), 4 Hz oscillations promote the cofiring of dmPFC and BLA neurons in the trough of the oscillation providing the conditions for fear expression.

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Karalis, N., Dejean, C., Chaudun, F. et al. 4-Hz oscillations synchronize prefrontal–amygdala circuits during fear behavior. Nat Neurosci 19, 605–612 (2016). https://doi.org/10.1038/nn.4251

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