Synaptic correlates of fear extinction in the amygdala

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as post-traumatic stress are characterized by an impaired ability to learn that cues previously associated with danger no longer represent a threat.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.

Fig. 1

Increased inhibition of CEm neurons in extinction and conditioned inhibition. (a) Experimental set-up. (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 values show % freezing during randomly selected 30-s periods. During the conditioning phase, all groups were presented with 4 CSts but they were paired with footshocks only in the “Fear conditioning” (black) and “Fear Conditioning plus extinguished” (red) groups. Nevertheless, for all groups, the data shows % time freezing during the CSt. During the extinction training phase, the “Fear conditioning” (black) and “Unpaired” (blue line and filled circles) groups were not presented with the CSt. Thus, we provide % time spent freezing during corresponding 30-s periods. 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 A BLA stimuli. (e) Intensity-dependence of BLA-evoked IPSPs in CEm neurons (average ± s.e.m.). Inset in e shows rising phase of BLA-evoked EPSPs (400 µA). Number of tested CEm cells: Fear conditioned 16; Extinguished 16; Naïve 12; Unpaired 10.

Fig. 2

Group-related differences in CEm EPSP slopes and orthodromic spiking in response to BLA stimulation. (a) Slope of BLA-evoked EPSPs (initial 2 ms; from −70 mV; average ± s.e.m.) and (b) percent BLA stimuli (400 µA) eliciting orthodromic spikes (average ± s.e.m.; from rest) in CEm cells from the various groups (x-axes). Inset, normalized frequency distribution of BLA-evoked spike latencies in CEm neurons of the fear conditioned group. (c–f) Representative examples of BLA-evoked responses (10 superimposed stimuli) in CEm cells from the various groups. Red arrows indicates average time of EPSP-IPSP transition in CEm cells from the extinction group studied at −45 mV.

Fig. 3

Increased recruitment of CEl neurons by BLA inputs in conditioned inhibition. (a) Representative examples of BLA-evoked responses in CEl cells in control aCSF. Superimposition of four responses elicited by BLA stimuli of 200–500 µA increasing in 100 A steps. (b) Intensity-dependence of BLA-evoked EPSP peak amplitudes in CEl neurons (average ± s.e.m.). Number of tested CEl cells: Fear conditioned 14; Extinguished 14; Naïve 13; Unpaired 14. (c) Slope of BLA-evoked (400 µA stimuli) EPSPs (first 2 ms) in CEl neurons from the various groups (average ± s.e.m.). Inset shows 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).

Fig. 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 shows 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.). (c) Percent BLA stimuli (400 µA) eliciting orthodromic spikes from rest (average ± s.e.m.) in ITC cells from the various groups (x-axis).

Fig. 5

Mechanisms underlying increased BLA responsiveness of ITC cells in extinction. (a) Histogram on left shows paired pulse ratio (average ± s.e.m.) in ITC cells from the control (n = 34) and extinction (n = 9) groups. Traces on right show representative examples of ITC responses to paired BLA stimuli (50 ms inter-stimulus interval; 500 µA). (b) Histogram on left shows Non-NMDA to NMDA ratio (average ± s.e.m.) in ITC cells from the control (n = 27) and extinction (n = 8) groups. Traces on right show representative examples of ITC responses to BLA stimuli (500 µA) at −80 and 55 mV.

Fig. 6

Infralimbic (IL) inactivation blocks extinction-related changes in the efficacy of BLA synapses onto ITC cells. (a) Experimental paradigm. (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 2. Inset shows extent of fluorophore-conjugated muscimol diffusion in the infralimbic cortex. (c) Non-NMDA-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.