Synaptic correlates of fear extinction in the amygdala

Journal name:
Nature Neuroscience
Volume:
13,
Pages:
489–494
Year published:
DOI:
doi:10.1038/nn.2499
Received
Accepted
Published online

Abstract

Anxiety disorders such as post-traumatic stress are characterized by an impaired ability to learn that cues previously associated with danger no longer represent a threat. However, the mechanisms underlying fear extinction remain unclear. We found that fear extinction in rats was associated with increased levels of synaptic inhibition in fear output neurons of the central amygdala (CEA). This increased inhibition resulted from a potentiation of fear input synapses to GABAergic intercalated amygdala neurons that project to the CEA. Enhancement of inputs to intercalated cells required prefrontal activity during extinction training and involved an increased transmitter release probability coupled to an altered expression profile of ionotropic glutamate receptors. Overall, our results suggest that intercalated cells constitute a promising target for pharmacological treatment of anxiety disorders.

At a glance

Figures

  1. Increased inhibition of CEm neurons in extinction and conditioned inhibition.
    Figure 1: Increased inhibition of CEm neurons in extinction and conditioned inhibition.

    (a) Experimental setup. EC, external capsule; LA, lateral amygdala; Rec, recording pipette. (b) Control and experimental groups. (c) Proportion of time spent freezing (average ± s.e.m.) during the various phases of the behavioral protocol (x axis). During habituation, no CSt was presented and the data shown are of freezing during randomly selected 30-s periods. During the conditioning phase, all groups were presented with four CSts, but they were paired with foot shocks only in the fear conditioning (black) and fear conditioning plus extinguished (red) groups. Nevertheless, the data shown represent the time spent freezing during the CSt for all groups. During the extinction training phase, the fear conditioning (black) and unpaired (blue line and filled circles) groups were not presented with the CSt. The time spent freezing during corresponding 30-s periods is shown. The fear conditioning plus extinguished (red) groups were presented with 20 CSts. (d) Representative examples of BLA-evoked responses in CEm cells recorded with 10 mM QX-314 in pipette solution. Three superimposed responses elicited by 300, 400 and 500 μA BLA stimuli are shown. (e) Intensity dependence of BLA-evoked IPSPs in CEm neurons (average ± s.e.m.). Inset, rising phase of BLA-evoked EPSPs (400 μA). We tested 16 fear-conditioned, 16 fear-extinguished, 12 naive and 10 unpaired CEm cells.

  2. Group-related differences in CEm EPSP slopes and orthodromic spiking in response to BLA stimulation.
    Figure 2: Group-related differences in CEm EPSP slopes and orthodromic spiking in response to BLA stimulation.

    (a,b) Slope of BLA-evoked EPSPs (initial 2 ms, from −70 mV, average ± s.e.m.; a) and percent BLA stimuli (400 μA) eliciting orthodromic spikes (average ± s.e.m., from rest; b) in CEm cells from the various groups (FC, fear conditioned; Nai, naive; Ext, fear conditioning plus extinguished; Unp, unpaired). Inset, normalized frequency distribution of BLA-evoked spike latencies in CEm neurons of the fear-conditioned group. (cf) Representative examples of BLA-evoked responses (ten superimposed stimuli) in CEm cells from the various groups. Red arrows indicate the average time of EPSP-IPSP transition in CEm cells from the extinction group studied at −45 mV.

  3. Increased recruitment of CEl neurons by BLA inputs in conditioned inhibition.
    Figure 3: Increased recruitment of CEl neurons by BLA inputs in conditioned inhibition.

    (a) Representative examples of BLA-evoked responses in CEl cells in control artificial cerebrospinal fluid (aCSF). Four responses elicited by 200–500-μA BLA stimuli, increasing in 100-μA steps, are superimposed. (b) Intensity dependence of BLA-evoked EPSP peak amplitudes in CEl neurons (average ± s.e.m.). We tested 14 fear-conditioned, 14 fear-extinguished, 13 naive and 14 unpaired CEl cells. (c) Slope of BLA-evoked (400 μA stimuli) EPSPs (first 2 ms) in CEl neurons from the various groups (average ± s.e.m.). Inset, rising phase of BLA-evoked EPSPs. (d) Percent BLA stimuli (400 μA) eliciting orthodromic spikes from rest (average ± s.e.m.) in CEl cells from the various groups (x axis).

  4. Enhanced efficacy of BLA synapses onto ITC cells in extinction.
    Figure 4: Enhanced efficacy of BLA synapses onto ITC cells in extinction.

    (a) Intensity dependence of BLA-evoked EPSPs in ITC neurons (average ± s.e.m.) in control aCSF. Inset, representative ITC cells from the extinction (red) and fear-conditioning (black) groups (300 μA). (b) Slope of BLA-evoked (400-μA stimuli) EPSPs (first 2 ms) in ITC neurons from the various groups (average ± s.e.m.). Rats from the U+CS group were treated similar to rats from the unpaired group except that they received 20 unpaired presentations of the CSt on day 3. (c) Percent BLA stimuli (400 μA) eliciting orthodromic spikes from rest (average ± s.e.m.) in ITC cells from the various groups (x axis).

  5. Mechanisms underlying increased BLA responsiveness of ITC cells in extinction.
    Figure 5: Mechanisms underlying increased BLA responsiveness of ITC cells in extinction.

    (a) Left, paired pulse ratio (average ± s.e.m.) in ITC cells from the control (n = 34) and extinction (n = 9) groups. Right, representative examples of ITC responses to paired BLA stimuli (50-ms inter-stimulus interval; 500 μA). (b) Left, nonNMDA to NMDA ratio (average ± s.e.m.) in ITC cells from the control (n = 27) and extinction (n = 8) groups. Right, representative examples of ITC responses to BLA stimuli (500 μA) at −80 and 55 mV.

  6. Infralimbic (IL) inactivation blocks extinction-related changes in the efficacy of BLA synapses onto ITC cells.
    Figure 6: Infralimbic (IL) inactivation blocks extinction-related changes in the efficacy of BLA synapses onto ITC cells.

    (a) Experimental procedure. (b) Intensity-dependence of BLA-evoked responses in ITC cells from the vehicle (n = 15) and muscimol (n = 11) groups (average ± s.e.m.). Dashed line indicates data from unpaired group reproduced from Figure 4. Inset, extent of fluorophore-conjugated muscimol diffusion in the infralimbic cortex. (c) NonNMDA to NMDA ratio (average ± s.e.m.) in ITC cells from the vehicle (n = 9) and muscimol (n = 8) groups. (d) Representative examples of ITC responses to BLA stimuli (500 μA) at −80 and 55 mV.

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Author information

  1. These authors contributed equally to this work.

    • Taiju Amano &
    • Cagri T Unal

Affiliations

  1. Center for Molecular and Behavioral Neuroscience, Rutgers, The State University of New Jersey, Newark, New Jersey, USA.

    • Taiju Amano,
    • Cagri T Unal &
    • Denis Paré

Contributions

T.A. and C.T.U. performed all of the electrophysiological experiments and most of the analyses on ITC and CEA cells, respectively. T.A. performed the behavioral training and C.T.U. scored the behavior. D.P. designed the experiments, wrote the paper and contributed to data analysis.

Competing financial interests

The authors declare no competing financial interests.

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