Persistence of fear memory across time requires the basolateral amygdala complex

Abstract

Mammals evolved a potent fear-motivated defensive system capable of single-trial fear learning that shows no forgetting over the lifespan of the animal. The basolateral amygdala complex (BLA) is considered an essential component of this conditional fear learning system. However, recent studies challenge this view and suggest that plasticity within other brain regions (i.e., central nucleus of the amygdala) may be crucial for fear conditioning. In the present study, we examine the mnemonic limits of contextual fear conditioning in the absence of the BLA using overtraining and by measuring remote fear memories. After excitotoxic lesions of the BLA were created, animals underwent overtraining and were tested at recent and remote memory intervals. Here we show that animals with BLA lesions can learn normal levels of fear. However, this fear memory loses its adaptive features: it is acquired slowly and shows substantial forgetting when remote memory is tested. Collectively, these findings suggest that fear-related plasticity acquired by brain regions outside of the BLA, unlike those acquired in the intact animals, do so for a relatively time-limited period.

Fig. 1.

Experimental design for experiment 1. Animals with sham or BLA lesions were overtrained for context fear and retention tested 1 or 30 days later.

Fig. 2.

Histological analysis of BLA or Sham lesions for Experiments 1 and 2. (A) Representative photomicrograph of coronal sections (4× magnification) through the amygdala stained with NeuN antibody: (i) lesion, (ii) sham. The dotted red line depicts the extent of the BLA (lateral and basal nuclei). (B) Experiment 1: Reconstruction of the minimal (red) and maximal (pink) excitotoxic lesion in the region of the BLA (AP 2.6 and 3.3 mm). Coronal section images of the brain taken and adapted from Swanson (40). (C) Experiment 2: Reconstruction of the minimal (red) and maximal (pink) excitotoxic lesion in the region of the BLA (AP 2.6 and 3.3 mm). Coronal section images of the brain taken and adapted from Swanson (40).

Fig. 3.

Behavioral measurements of Fear Conditioning in sham or BLA lesioned animals, during 76 trial fear acquisition and a test context fear memory retention at 1 or 30 days after overtraining.(A) Acquisition of conditional fear as indexed by percent time observed freezing. Mean (±SEM) percentage of observations spent freezing during a 76-footshock contextual fear conditioning session. (B) Total mean (±SEM) percentage of observations spent freezing during an 8-min test session for conditional fear.

Fig. 4.

Experimental design for experiment 2. Animals with sham or BLA lesions were overtrained or single-trial trained with context fear tested 7 and 30 days later. BLA-lesioned animals were given an additional footshock in the original overtrained context (A) or in a novel context (B), and the next day were returned to the same context to measure savings or sensitization, respectively.

Fig. 5.

Total mean (±SEM) percentage of observations spent freezing during an 8-min test session for conditional fear. BLA RET, BLA-lesioned group with retention test at 7 or 30 days; SHAM RET, sham group with retention test at 7 or 30 days; BLA SAV, BLA-lesioned group with saving tested 2 days following retention test at 7 or 30 days; BLA SENS, BLA-lesioned group with sensitization tested 2 days following retention test at 7 or 30 days.)

Fig. 6.

Savings (SAV) and sensitization (SEN) mean scores as derived from subtracting the percent freezing between retention test and savings or sensitization test.