Synaptic encoding of fear extinction in mPFC-amygdala circuits

Abstract

Retrieval of fear extinction memory is associated with increased firing of neurons in the medial prefrontal cortex (mPFC). It is unknown, however, how extinction learning-induced changes in mPFC activity are relayed to target structures in the amygdala, resulting in diminished fear responses. Here, we show that fear extinction decreases the efficacy of excitatory synaptic transmission in projections from the mPFC to the basolateral nucleus of the amygdala (BLA), whereas inhibitory responses are not altered. In contrast, synaptic strength at direct mPFC inputs to intercalated neurons remains unchanged after extinction. Moreover, priming stimulation of mPFC projections induced heterosynaptic inhibition in auditory cortical inputs to the BLA. These synaptic mechanisms could contribute to the encoding of extinction memory by diminishing the ability of projections from the mPFC to drive BLA activity while retaining the ability of intercalated neurons to inhibit the output nuclei of the amygdala.

Figure 1

Optogenetic Projection-Specific Activation of mPFC Inputs to Amygdala (A) Top, experimental design. A vertical line indicates location of the coronal section shown below. Bottom, expression of eYFP-tagged ChR2 in the mPFC (green). PL and IL, prelimbic and infralimbic divisions of the mPFC, respectively; DP, dorsal peduncular cortex; fmi, forceps minor of the corpus callosum; ac, anterior commissure. (B) Left, ChR2-mediated currents in IL/mPFC neuron were evoked by 1-s pulses of light (470 nM, blue horizontal bar) with variable intensities (0.02-0.61 mW/mm²) at −80 mV in voltage-clamp mode in the presence of NBQX (10 μM) and bicuculline (30 μM). Right, input-output curves of ChR2-mediated photocurrents showing peak (filled circles) and sustained current components (open circles) plotted as a function of light intensity (light power density, mW/mm², n = 4 neurons). (C) Left, action potentials (AP) were evoked in a pyramidal neuron in the mPFC in current-clamp mode by 1-ms pulses of photostimuli at different frequencies (blue vertical lines). Right, summary plot of AP firing probability versus photostimulation frequency (n = 5 neurons). (D) Top, experimental design. A vertical line indicates location of the coronal section through the amygdala. Bottom, microscopic images showing afferent fibers from the mPFC terminating in the BLA (green). BLAa and BLAp are anterior and posterior divisions of the BLA. dITC and vITC are dorsal and ventral clusters of ITC neurons. Red fluorescence indicates fluorescent Nissl stain. (E) ChR2-eYFP-expressing mPFC fibers in the amygdala. (F) Experimental setup for recording of photostimulation-induced synaptic responses. (G) Traces of lightevoked EPSCs recorded in a BLA principal neuron at −80 mV, blocked by NBQX (10 μM). (H) EPSCs were also blocked by TTX (1 μM). (I) Proportions of neurons in different nuclei of the amygdala which displayed EPSCs upon photostimulation of mPFC fibers. Numbers of neurons examined were 13, 215, 82, 146, 19, and 33 for LA, BLAa principal neurons (PN), BLAa interneurons (IN), ITC, CeL, and CeM, respectively. Error bars are SEM.

Figure 2

Extinction of Conditioned Fear Is Associated with Reduction of Excitatory Synaptic Strength in mPFC-BLA Pathway (A) Design of fear conditioning and extinction experiments (CS-only group, n = 16 mice; CS-US, n = 22 mice; fear extinction, n = 21 mice). (B) Freezing responses (% freezing) to the CS during different phases of the behavioral paradigm (fear conditioning, extinction and test sessions). (C) Freezing scores on the test day (Day 3). For each mouse tested, freezing scores to two CSs were averaged. (D) Experimental design. BLA/PN, BLA principal neuron. (E) Resting membrane potential (RMP, left) and input resistance (R_in, right) in BLA principal neurons in different groups. R_in was calculated as in Figure S6A. (F) Action potential (AP) frequencies in BLA/PN induced by 1-s current injections at −85 mV in current-clamp mode. Inset shows AP firing in BLA/PN (scale bars: 0.4 s, 40 mV). n = 20 cells from 5 mice in each group for both E and F. (G) EPSCs (averages of 5 responses) in the mPFC-BLA pathway in mice from all groups. EPSCs were induced by photostimuli of three different intensities. (H) Input-output curves for peak amplitudes of EPSCs in the mPFC-BLA pathway. p < 0.0001 for EPSC amplitudes in fear extinction group (n = 61 cells from 15 mice) versus CS-only group (n = 54 cells from 10 mice) or versus CS-US mice (n = 61 cells from 15 mice). (I) A distribution of peak amplitudes of EPSCs evoked by photostimulation (12.5 mW/mm²) in the mPFCBLA pathway. A fitted Gaussian curve (blue) is overlaid on the histogram. EPSCs were blocked by NBQX (10 μM) and D-AP5 (50 μM) (inset, scale bars: 20 ms, 0.5 nA). (J) Traces of AP firings induced by photostimulation of mPFC projections (5-ms pulses, blue bars). APs were recorded in BLA/PN in cell-attached mode. (K) Probability of AP firing induced by photostimulation of mPFC projections. AP probability at each light intensity was determined after 5-10 trials. (L) The latency to AP firing versus photostimulation intensity. n = 13-14 cells from 5 mice in each group for both K and L. Error bars are SEM.

Figure 3

Mechanisms of Fear Extinction-Associated Depression of Synaptic Transmission in mPFC-BLA Pathway (A) EPSCs evoked in BLA principal neurons by paired photostimuli (2.5 mW/mm²; 50 ms interpulse interval) in slices from all groups. (B) PPR in extinction group (n = 22 cells from 10 mice) was larger than in CS-only (n = 21 cells from 7 mice) or CS-US groups (n = 28 cells from 9 mice). (C) Photostimulation-induced EPSCs in a BLA principal neuron at holding potentials ranging from −80 mV to +40 mV in the presence of bicuculline (30 μM). (D) Current-voltage (I-V) plots of EPSCs in the mPFC-BLA pathway. Left, the amplitude of AMPAR EPSC was measured at the peak (marked with a filled circle in C). Right, the amplitude of NMDAR EPSCs was measured at 50 ms after the photostimulus (marked with an open circle in C). AMPAR-EPSCs and NMDAR EPSCs at different holding potentials were normalized to EPSCs recorded at −80 mV or +40 mV, respectively (I_norm). n = 15, 24 and 26 cells from 6, 8, and 8 mice for CS-only, CS-US, and fear extinction groups, respectively. (E) AMPAR/NMDAR EPSC amplitude ratios, calculated as the ratio of peak amplitude of AMPAR EPSC at −80 mV to NMDAR EPSC amplitude at +40 mV. Small symbols show data from individual neurons whereas larger circles represent average values. (F) Top, a protocol for the induction of LTD. To induce LTD, the recorded BLA principal neuron was depolarized from a baseline holding potential to −65 mV and presynaptic mPFC inputs were photostimulated (1 ms) at 1 Hz for 10 min (black bar). Bottom, time course of EPSC amplitude changes. Traces are the averages of 10-15 EPSCs recorded before (1) and 25-30 min after LTD induction (2). n = 16 neurons from 12 mice. (G) LTD was blocked when BAPTA (10 mM) was included in the pipette solution (n = 6 neurons from 4 mice). (H) Summary of LTD experiments as in (F) and (G). (I) mPFC-BLA EPSCs evoked in BLA neurons by paired photostimuli (50-ms interval) before (1) and after LTD induction (2). Below, the time course of PPR changes (normalized to the average baseline PPR). n = 8 neurons from 7 mice. (J) Left, PPR changes after the induction of LTD. n = 8 neurons from 7 mice. Right, PPR changes correlated positively with the magnitude of LTD. Error bars are SEM.

Figure 4

Activation of mPFC Afferents Leads to Feed-Forward Inhibition of BLA Principal Neurons (A) Experimental design for recording of light-evoked (blue) synaptic responses in local circuit interneurons (IN) in the BLA. (B) BLA interneuron exhibits non-accommodating AP firing (inset). Synaptic responses were induced in BLA interneuron by photostimulation of mPFC projections in current clamp mode with stronger light intensities resulting in AP firing (red trace). (C) Probability of AP firing in BLA interneurons in response to photostimulation of the mPFC projections. It was assayed in cell-attached or current-clamp modes at resting membrane potential (n= 10 neurons). (D) Experimental design for recording of the light-evoked EPSC/IPSC in BLA principal neurons (BLA/PN). (E) The EPSC/IPSC in a BLA/PN at holding potentials ranging from −80 mV to 0 mV. (F) Synaptic currents recorded at 0 mV were completely blocked by GABA_a receptor antagonist, bicuculline (30 μM, Bic; red trace at 0 mV). However, the peak amplitude of synaptic currents at −80 mV was not affected by bicuculline (red trace at −80 mV). (G) Both EPSC and IPSC (recorded at −80 mV or 0 mV, respectively) were blocked by jointly-applied NBQX (10 μM) and D-AP5 (50 μM) (red traces at −80 mV and 0 mV). (H) Left, synaptic latency analysis for EPSCs and IPSCs recorded in BLA/PN. Right, synaptic latency of IPSCs in the mPFC-BLA pathway (n = 19 cells) was significantly longer compared to photostimulation-induced EPSCs (n = 22 cells) or EPSCs induced by electrical stimulation of inputs to the BLA from the auditory cortex (AC, n = 21 cells). (I) Left, photostimulation-induced EPSCs in a BLA/PN (12.5 mW/mm², 1-ms duration) under control conditions (black trace) and in the presence of bicuculline (30 μM, red trace). Right, average total charge transfer and 90%-to-10% decay time of EPSCs recorded in the presence or absence of bicuculline (n = 6 neurons from 4 mice). Error bars are SEM.

Figure 5

Fear Extinction Is Not Associated With Changes in Feed-forward Inhibitory Circuits in the mPFC-BLA Pathway (A) Design for recording EPSCs in the mPFC-BLA interneuron (BLA/IN) pathway. (B) Resting membrane potential (RMP) and input resistance in BLA/IN. Input resistance was calculated as in Figure S6A. (C) AP firing frequencies in BLA/IN in different groups. AP firing was induced as in Figure 2F. Inset, an example of AP firing (scale bars: 0.4 s, 40 mV). n = 20, 26, and 24 cells from 5 mice each in CS-only, CS-US, and fear extinction groups, respectively for both B and C. (D) EPSCs in BLA/IN at −85 mV, induced by photostimulation of mPFC projections at different light intensities (blue bar). (E) Peak amplitudes of monosynaptic EPSCs in BLA/IN versus photostimulation intensity. n = 14, 16, and 17 cells from 5 mice each in CS-only, CS-US, and fear extinction groups, respectively. (F) Traces of AP firings evoked by photostimulation of mPFC projections (1-ms-long) at different light intensities. AP firings were recorded in BLA/IN in cell-attached mode or in current-clamp mode at resting membrane potential. (G) A plot of AP firing probability versus photostimulation intensity. AP probability at each photostimulation intensity was calculated following 5 trials. n = 14, 12, and 10 cells from 7, 5, and 4 mice for CS-only, CS-US, and fear extinction groups, respectively. (H) Experimental design for recording of IPSCs in the BLA/IN to BLA/PN pathway. IPSCs in BLA/PN were induced by electrical stimulation with an electrode placed onto the BLA in the presence of NBQX (10 μM) and D-AP5 (50 μM), and were recorded at 0 mV in voltage-clamp mode. Cs⁺-based pipette solution with high [Cl^-]_in (132 mM) was used in experiments shown in panels H-K. The IPSC (black trace, inset) was blocked by 30 μM bicuculline (red trace). Scale bars: 20 ms, 50 pA. (I) I-V plots for IPSCs (evoked as in H) were similar in all groups. Inset, IPSCs in the BLA/IN-BLA/PN pathway at V_h ranging from −35 mV to +5 mV (scale bars: 20 ms, 100 pA). n = 9-11 cells from 3-4 mice in each group. (J) IPSCs in the BLA/IN-BLA/PN pathway at different stimulation intensities. IPSCs were induced by electrical stimulation of the BLA, and recorded in BLA/PN at 0 mV in voltage-clamp mode. (K) A plot of peak amplitudes of IPSCs in the BLA/IN-BLA/PN pathway versus stimulation intensity. n = 16-18 cells from 5 mice in each group. (L) Experimental design for recording of feed-forward inhibition in the mPFC-BLA/IN-BLA/PN pathway. (M) IPSCs recorded in BLA/PN in different groups. IPSCs were recorded in BLA/PN at 0 mV in voltage-clamp mode with Cs⁺-based pipette solution containing high [Cl-]_in (132 mM). (N) A plot of peak amplitudes of feed-forward IPSCs (as in M) versus photostimulation intensity. The time window for detection of peak amplitudes was set as the time interval between 8 and 12 ms after the onset of photostimulation (see Figures S7E-S7H for details). n = 15-16 cells from 4 mice in each group. Error bars are SEM.

Figure 6

Priming Stimulation of mPFC Fibers Induces Heterosynaptic Inhibition in the AC-BLA Pathway (A) Experimental design. Afferent fibers originating in the auditory cortex (AC) were stimulated electrically with an electrode placed onto the external capsule (EC), whereas ChR2-expressing mPFC fibers were stimulated with blue light. Synaptic responses were recorded in BLA/PN. (B) Co-activation of mPFC and AC fibers. Three priming photostimuli (50-ms interpulse intervals), delivered to mPFC fibers, preceded electrical stimulation of the AC-BLA pathway with variable time intervals (△t). Lower traces show monosynaptic EPSCs and disynaptic IPSCs recorded in a BLA neuron at −80 mV or 0 mV, respectively, during photostimulation of mPFC fibers. (C) Traces of EPSCs evoked by stimulation of the AC-BLA pathway with or without priming photostimulation of mPFC fibers (red and black traces, respectively) with variable △t (200-800 ms). (D) PPR (50-ms interval) was increased in the AC-BLA pathway during heterosynaptic inhibition. (E) EPSC amplitude inhibition as a function of △t. mPFC priming induced significant inhibition of EPSCs with △t = 0.8 ≤ (n = 48 neurons). (F) PPR changes after mPFC priming (normalized to no priming control PPR). PPR was increased during mPFC priming-induced inhibition of EPSCs (n = 48 neurons). (G) Correlation plot of the magnitude of heterosynaptic inhibition versus PPR changes (n = 48 neurons). (H) Correlation plot of the magnitude of heterosynaptic inhibition versus peak amplitudes of priming IPSCs (n = 39 neurons). (I) EPSCs in the AC-BLA pathway recorded 400 ms after priming stimulation of mPFC fibers (red traces) or without mPFC priming (black traces) in control external solution (left) or in the presence of CGP 52432 (10 μM, right). (J) EPSC inhibition (%) in the AC-BLA pathway after mPFC priming under control conditions or in the presence of CGP 52432 (n = 6 neurons). (K) PPR changes after mPFC priming with or without CGP 52432 in the external solution (n = 6 neurons). (L) Heterosynaptic inhibition in the AC-BLA pathway with △t = 400 ms in mice from CS-only, CS-US and fear extinction groups. The intensity of priming photostimulation of the mPFC fibers was adjusted to generate EPSCs of the similar amplitudes in all three behavioral groups (no difference in the EPSC amplitudes between the groups, p = 0.91). (M) Summary plots of heterosynaptic inhibition (n = 21, 18, and 19 cells from 6, 8, and 7 mice for CS-only, CS-US, and fear extinction groups, respectively). (N) Same as in M but for PPR data. Error bars are SEM.

Figure 7

Direct Activation of Intercalated Neurons by Projections from the mPFC (A) ChR2-eYFP-expressing mPFC fibers (green) in dorsolateral (dITC, left) and ventromedial clusters of ITC neurons (vITC, right). Putative synaptic contacts are indicated as arrowheads. Ast, amygdalostriatal transition. (B) Left, EPSCs in an intercalated neuron at −80 mV evoked by photostimulation of mPFC fibers. EPSCs were inhibited by NBQX (10 μM). Right, synaptic latencies of EPSCs in ITC neurons or BLA neurons (n = 21, 17, and 9 cells for BLA/PN, dITC, and vITC, respectively). (C) EPSCs in the mPFC-ITC pathway in mice from each group induced by photostimuli of increasing intensity. EPSCs were recorded at −80 mV. (D) Input-output curves of EPSCs in mPFC-dITC (left) and mPFC-vITC pathways (right) (mPFC-dITC: n = 20-33 cells from 7-10 mice per group; mPFC-vITC: n = 8-9 cells from 3-4 mice per group). (E) EPSCs in the mPFC-ITC pathway at holding potentials from −80 to +40 mV in the presence of bicuculline (30 μM). (F) I-V plots of EPSCs in the mPFC-ITC pathway. AMPAR- and NMDAR-mediated EPSCs were measured and normalized as in Figure 3D (data from both ITC clusters were pulled together; n = 8-11 cells from 5-7 mice per each group). (G) AMPAR/NMDAR EPSC amplitude ratios in the mPFC-ITC pathway in different behavioral groups. (H) Sparse innervation of the CeA by ChR2-eYFP-expressing mPFC fibers (green). CeL and CeM, lateral and medial divisions of the CeA, respectively. st, the stria terminalis. (I) Left, EPSC (black) and IPSC (red) recorded in a CeM neuron in voltage-clamp mode at −80 mV and 0 mV, respectively. Inset shows spiking response of a CeM neuron to depolarizing current injection. Right, IPSCs had longer latencies compared to EPSCs (n = 13 and 30 neurons for EPSCs and IPSCs, respectively; p < 0.0001). (J) A summary plot of EPSC/IPSC ratios in BLA/PN, ITC neurons and CeM neurons (n = 21, 9, and 28 cells for BLA/PN, ITC neurons, and CeM neurons, respectively). Error bars are SEM.

Figure 8

Modulatory Roles of the mPFC-Amygdala Pathways in Fear Extinction After fear extinction, excitatory synaptic efficacy in the mPFC projections to principal neurons in the BLA (BLA/PN) decreases (1), resulting in a reduced balance between excitation and inhibition in this pathway. Projections from the mPFC also activate local GABAergic interneurons (BLA/IN, 2), which, in turn, induce heterosynaptic inhibition of the AC-BLA pathway, an auditory CS input. Synaptic efficacy in direct mPFC inputs to intercalated neurons remains unchanged following extinction training (3). Therefore, increased activity of the mPFC during recall of extinction memory (Milad and Quirk, 2002; Holmes et al., 2012) would induce more efficient heterosynaptic inhibition in the AC-BLA pathway and more frequent activation of ITC neurons, resulting in robust inhibitory responses in the CeA (specifically, CeM), the output nucleus of the amygdala. Through these mechanisms, the mPFC could attenuate the spiking output of the amygdala during the CS presentation, thus suppressing the conditioned fear response after fear extinction.