Dissociable roles for the ventromedial prefrontal cortex and amygdala in fear extinction: NR2B contribution

Abstract

Fear extinction, which involves learning to suppress the expression of previously learned fear, requires N-methyl-D-aspartate receptors (NMDARs) and is mediated by the amygdala and ventromedial prefrontal cortex (vmPFC). Like other types of learning, extinction involves acquisition and consolidation phases. We recently demonstrated that NR2B-containing NMDARs (NR2Bs) in the lateral amygdala (LA) are required for extinction acquisition, but whether they are involved in consolidation is not known. Further, although it has been shown that NMDARs in the vmPFC are required for extinction consolidation, whether NR2Bs in vmPFC are involved in consolidation is not known. In this report, we investigated the possible role of LA and vmPFC NR2Bs in the consolidation of fear extinction using the NR2B-selective antagonist ifenprodil. We show that systemic treatment with ifenprodil immediately after extinction training disrupts extinction consolidation. Ifenprodil infusion into vmPFC, but not the LA, immediately after extinction training also disrupts extinction consolidation. In contrast, we also show pre-extinction training infusions into vmPFC has no effect. These results, together with our previous findings showing that LA NR2Bs are required during the acquisition phase in extinction, indicate a double dissociation for the phase-dependent role of NR2Bs in the LA (acquisition, not consolidation) and vmPFC (consolidation, not acquisition).

Figure 1.

Fear extinction requires posttraining NR2B activation. (a) Percent freezing during conditioning (first trial and averages of 2–4 and 5–7 tone–footshock trials on Day 1), trial-by-trial extinction training (20 tone-alone trials on Day 2), and extinction test (5 tone-alone trials on Day 3). Vehicle (white circles) or ifenprodil (black circles) injections were administered immediately after extinction training (arrow); No-Ext control group (No-Ext; gray triangles). (b) Averaged percent freezing across all extinction test trials for vehicle-treated (white bar), ifenprodil-treated (black bar), and No-Ext (gray bar) rats. *P < 0.05 relative to control groups (vehicle and No-Ext) in the same session. +P < 0.05 relative to ifenprodil and vehicle groups in the same session.

Figure 2.

Fear extinction does not require posttraining NR2B activation in the LA. (a) Coronal drawings show the localization of injector tips (top to bottom relative to bregma: −2.80 mm and −3.60 mm [adapted from Paxinos and Watson 1998]) from rats infused with vehicle (white circles) or ifenprodil (1 μg; black circles). B: basal amygdala; CE: central amygdala. (b) Percent freezing during conditioning (first trial and averages of 2–4 and 5–7 tone–footshock trials on Day 1), trial-by-trial extinction training (20 tone-alone trials on Day 2), and extinction test (5 tone-alone trials on Day 3). Vehicle (white circles) and ifenprodil (black circles) injections were administered immediately after extinction training (arrow); No-Ext control group (gray triangles). (c) Averaged percent freezing across all extinction retention trials for vehicle-treated (white bar), ifenprodil-treated (black bar), and No-Ext (gray bar) rats. +P < 0.05 relative to ifenprodil and vehicle groups in the same session.

Figure 3.

Fear extinction requires posttraining NR2B activation in the vmPFC. (a) Coronal drawings show the localization of injector tips (top to bottom relative to bregma: +3.20 mm and +2.70 mm (adapted from Paxinos and Watson 1998) from rats infused with vehicle (white circles) or ifenprodil (2 μg; black circles). IL: infralimbic cortex; PrL: prelimbic cortex. (b) Percent freezing during conditioning (first trial and averages of 2–4 and 5–7 tone–footshock trials on Day 1), trial-by-trial extinction training (20 tone-alone trials on Day 2), and extinction test (5 tone-alone trials on Day 3). Vehicle (white circles) and ifenprodil (black circles) injections were administered immediately after fear extinction (arrow); No-Ext control group (No-Ext; gray triangles). (c) Averaged percent freezing across all extinction retention trials for vehicle-treated (white bar), ifenprodil-treated (black bar), and No-Ext (gray bar) rats. *P < 0.05 relative to control groups (vehicle and No-Ext) in the same session. +P < 0.05 relative to ifenprodil and vehicle groups in the same session.

Figure 4.

Fear extinction does not require NR2B activation in the vmPFC during training. (a) Coronal drawings show the localization of injector tips (top to bottom relative to bregma: +3.20 mm and +2.70 mm (adapted from Paxinos and Watson 1998) from rats infused with vehicle (white circles), ifenprodil (2 μg; gray circles). IL: infralimbic cortex; PrL: prelimbic cortex. (b) Percent freezing during conditioning (first trial and averages of 2–4 and 5–7 tone-footshock trials on Day 1), trial-by-trial extinction training (20 tone-alone trials on Day 2), and extinction test (5 tone-alone trials on Day 3). Vehicle (white circles), ifenprodil (black circles) injections were administered before extinction training (arrow). (c) Averaged percent freezing across all extinction training and extinction retention trials for vehicle-treated (white bars), and ifenprodil-treated (black bars) rats.

Figure 5.

Model for extinction learning phases: NR2B contribution. Extinction acquisition (within-session): the lateral amygdala (LA) gradually encodes extinction via NR2B-containing NMDARs, leading to gradual decrease in fear response. Extinction consolidation (between-session): the vmPFC encodes extinction learning offline via NMDARs (including NR2Bs), instructed in part by amygdala input. Extinction retrieval: events that induced plasticity (“extinction memory trace”; asterisk) during and after extinction training are engaged in ventromedial prefrontal cortex (vmPFC)-induced suppression of amygdala output, leading to rapid reduction of fear responses. Ext: extinction; B: basal amygdala; CE: central amygdala.