Role of the basolateral amygdala in the storage of fear memories across the adult lifetime of rats

Abstract

The basolateral amygdala (BLA) is intimately involved in the development of conditional fear. Converging lines of evidence support a role for this region in the storage of fear memory but do not rule out a time-limited role in the memory consolidation. To examine this issue, we assessed the stability of BLA contribution to fear memories acquired across the adult lifetime of rats. Fear conditioning consisted of 10 tone-shock pairings in one context (remote memory), followed 16 months later by 10 additional tone-shock pairings with a novel tone in a novel context (recent memory). Twenty-four hours after recent training, rats were given NMDA or sham lesions of the BLA. Contextual and tone freezing were independently assessed in individual test sessions. Sham-lesioned rats showed high and comparable levels of freezing across all context and tone tests. In contrast, BLA-lesioned rats displayed robust freezing deficits across both recent and remote tests. Subsequent open-field testing revealed no effects of BLA lesions on activity patterns in a dark open field or during bright light exposure. Lesioned rats were able to reacquire normal levels of context-specific freezing after an overtraining procedure (76 unsignaled shocks). Together, these findings indicate that BLA lesions do not disrupt freezing behavior by producing hyperactivity, an inability to suppress behavior, or an inability to freeze. Rather, the consistent pattern of freezing deficits at both training-to-lesion intervals supports a role for the BLA in the permanent storage of fear memory.

Figure 1.

Representative photomicrograph of BLA from sham (A) and lesioned (B) rats. Substantial cell loss in lesioned rats was apparent throughout the BLA. CEA, Central nucleus of the amygdala; PR, perirhinal cortex.

Figure 2.

Schematic diagram of sample behavioral procedure. Rats were given pavlovian fear conditioning in one context (remote training), followed 16 months later by additional training in a novel context (recent training). Lesions of the BLA were performed 1 d after recent training. After recovery, rats were given independent test sessions to assess remote and recent, context and tone memory. The exact contexts and test orders used were counterbalanced

Figure 3.

Remote and recent contextual fear. Each test consisted of an 8 min context exposure. No tones or shocks were delivered during these sessions. A, Remote contextual fear. When exposed to the remotely trained context, sham rats exhibited high levels of freezing across the 8 min extinction test. In contrast, BLA lesions produced near complete deficits in freezing. B, Recent contextual fear. Sham rats exhibited levels of freezing comparable with those observed in the remote context. Again, BLA-lesioned rats displayed significant deficits in freezing across the extinction test.

Figure 4.

Remote and recent tone fear. Tone tests were conducted by placing the rats in a novel context for a 2 min baseline period (BL), followed by a 6 min continuous tone presentation. A, Remote tone fear. Both sham- and BLA-lesioned rats displayed low levels of freezing during the 2 min baseline period. Whereas sham rats displayed high levels of fear during the tone presentation, BLA rats showed pronounced deficits across the test. B, Recent tone fear. Baseline freezing in sham- and BLA-lesioned rats was negligible. Sham rats displayed high levels of freezing across the tone test, and BLA rats again showed significant freezing deficits.

Figure 5.

General activity assessment. Rats were placed in a novel open field dimly illuminated by a 25 W red bulb. Crossovers were scored for 4 min. Two bright lights (100 W) illuminated the open field for the final 4 min of the test. Sham- and BLA-lesioned rats showed comparable levels of activity and rates of habituation throughout both phases of the test.

Figure 6.

Overtraining. BLA-lesioned rats were returned to one of the two original training contexts and given an overtraining procedure consisting of 76 unsignaled shocks. Over the next 2 d, rats were given 8 min extinction tests in the overtrained context and a novel context. Test order was counterbalanced. Data represents the mean of each 8 min extinction tests. Freezing data for sham rats tested in the same context is provided for comparison.