Post-training intra-basolateral amygdala infusions of norepinephrine enhance consolidation of memory for contextual fear conditioning

Abstract

Post-training infusions of drugs, including noradrenergic agonists and antagonists, into the basolateral amygdala (BLA) influence the consolidation of memory for training in several tasks, including inhibitory avoidance. There is, however, conflicting evidence concerning whether post-training intra-BLA drug infusions modulate the consolidation of contextual fear conditioning (CFC). In the present study, norepinephrine (NE) was infused bilaterally into the BLA of male Sprague Dawley rats immediately after training on two CFC tasks: a Y-maze and a straight alley. Post-training intra-BLA infusions enhanced memory of CFC training in the Y-maze, as assessed by percentage of time spent freezing and shock arm entrance latencies. Post-training intra-BLA infusions of NE enhanced 48 hr retention of CFC training in the straight alley, as assessed by shock compartment entrance latencies and the number of shocks required to learn to avoid entering the shock compartment. These findings indicate that the consolidation of memory for CFC, like that for inhibitory avoidance training, is influenced by post-training neuromodulatory influences within the BLA. Thus, the findings provide additional evidence consistent with the hypothesis that the BLA has a general role in modulating memory consolidation.

Figure 1.

A, Diagram of rat basolateral amygdala and adjacent structures (Paxinos and Watson,1997). B, Representative photomicrograph of needle track terminating in the BLA. Only data from animals that had needle tracks terminating in the BLA and had no lesions in the surrounding BLA tissue were included in the analyses.

Figure 2.

Enhanced retention of rats that received immediate post-training NE infusions into the BLA for Y-maze CFC. Groups are as follows (from left to right): vehicle shock (white bars; n = 10), 0.3 μg NE shock (hatched bars; n = 8), 1.0 μg NE shock (black bars; n = 10), vehicle no shock (white bars; n = 7), and 1.0 μg NE no shock (black bars; n = 10). A, Mean percentage of time (± SEM) spent freezing during the 8 min retention test on day 3. B, Mean latencies (± SEM), in seconds, to first entry into the shock arm during retention test. C, Mean latencies without freezing (± SEM), in seconds, during the 8 min retention test on day 3. ^*p < 0.005 compared with shocked vehicle controls and both no-shock controls.

Figure 3.

Enhanced retention of rats that received immediate post-training NE infusions into the BLA for straight-alley CFC. Groups are as follows: Vehicle-PBS (white bars; n = 15) and 1.0 μg NE (black bars; n = 16) A, Mean latencies, in seconds (± SEM), to enter the shock compartment during the retention test. B, Mean number of shocks (± SEM) administered for animal to remain in the safe compartment for 200 sec. ^*p < 0.05 compared with the vehicle controls.