Unilateral storage of fear memories by the amygdala

Abstract

Pavlovian fear conditioning is an associative learning task in which subjects are trained to respond defensively to a neutral conditioned stimulus (CS) by pairing it with an aversive unconditioned stimulus (US). This type of learning depends critically on the amygdala, and evidence suggests that synaptic plasticity within the lateral nucleus of the amygdala (LA) may be responsible for storing memories of the CS-US association. In the present study, we trained rats to fear an auditory CS by pairing it with a shock US delivered to one eyelid. Conditioning was assessed by measuring freezing responses evoked by the CS during a subsequent test session. The amygdala was unilaterally inactivated during either the training or the testing session by intracranial infusions of muscimol into the LA. We found that both acquisition and expression of conditioned freezing to the CS depended on the amygdala contralateral but not ipsilateral from the eyelid where the shock US was delivered. To explain this surprising result, we propose that the shock US is relayed from the eyelid to the amygdala via lateralized nociceptive sensory pathways, which causes memories of the CS-US association to be stored by the amygdala contralateral but not ipsilateral from the shocked eyelid. Our results demonstrate that the fear-learning circuitry of the amygdala is functionally lateralized according to the anatomical source of predicted threats. In future studies, the cellular mechanisms of emotional memory storage might be pinpointed by identifying cellular processes that occur only in the amygdala contralateral but not ipsilateral from the US during lateralized fear conditioning.

Figure 1.

Histological analysis of cannula placements. Locations of cannula tip placements where muscimol was delivered are shown (control placements where no drugs were delivered are omitted for clarity). All placement hemispheres are specified relative to the eyelid where the shock US was delivered during training. Squares indicate unilateral groups (IPSI/CONTRA), circles indicate the BI group, and asterisks indicate the PAIR group.

Figure 4.

Off-site control injections. a, Asterisks mark the site of muscimol injections into the caudate nucleus in the hemisphere contralateral from the shocked eyelid. b, Design of the experiment. c, Mean freezing scores for baseline and two test sessions. TEST1 was conducted drug-free 1 d after training with muscimol, and TEST2 was conducted with muscimol 1 d after drug-free training.

Figure 2.

Acquisition. a, Design of the acquisition experiment. b, Mean freezing scores (seconds of freezing) during the 20 s CS and pre-CS periods for each group of rats during the six trials of the baseline session. c, Mean freezing scores during the six trials of the expression test session. d, Mean freezing attributable specifically to the CS and not the training context (CS minus pre-CS freezing; see Results). Significance levels for Fisher's LSD tests: ^****p < 0.0001; ^***p < 0.001; n.s., not significant.

Figure 3.

Expression. a, Mean freezing score during the 20 s CS period on each day of the experiment. The subjects in the expression experiment were the rats from the paired group in the acquisition experiment, so days 1 and 2 are the acquisition experiment days. Days 3-8 are overtraining days (see Results), and days 9-15 are expression test days. Shading denotes days on which different amygdala hemispheres were inactivated before the test session. b, Pooled freezing scores for rats in both eyelid groups (left and right combined) during the pre-CS and CS periods for each of the four amygdala inactivation conditions of the expression test.