Retrograde amnesia for the stress-induced impairment of extinction: time-dependent and not so forgotten

Abstract

We investigated whether retrograde amnesia for the stress-induced impairment of extinction retrieval shares similar characteristics with original acquisition memories. The first experiment demonstrated that cycloheximide administered shortly after a single restraint stress session alleviated the impairment of extinction retrieval but not when administered following a longer delay (i.e., the amnesia for stress is time-dependent). A second experiment showed that the retrograde amnesia for stress could be alleviated by a second brief exposure to the stressor. These results demonstrating that amnesia for stress shares characteristics similar to original memories are explained using a retrieval-based memory integration model of retrograde amnesia.

Figure 1.

Mean (±SEM) latency (in seconds) to cross to the black side of the black–white shuttle box for all groups in experiment 1. Open bars represent the no stress control groups. Darker slashed bars represent the stressed experimental groups. Long latency scores for the No Ext, 0-saline, and 120-CHX groups represent significant fear. Decreased latency scores for the group that did not receive stress before training and extinction (Ext) represent significantly less avoidance (i.e., reduction of fear). Increased latency scores for the group that received stress 48 h before training (0-saline) compared with the no stress extinction group (Ext) represent the stress-induced impairment of extinction. The 0-CHX group's short latencies replicate retrograde amnesia for the stress-induced impairment of extinction. Significantly longer latencies for the group that received cycloheximide 2 h after stress (120-CHX) compared with the shorter latencies for the group that received cycloheximide immediately after stress (0-CHX) represent a temporal gradient of amnesia.

Figure 2.

Mean (±SEM) latency (in seconds) to cross to the black side of the black–white shuttle box for both groups in experiment 2. Longer latency scores for the stress group that was stressed 48 h prior to training replicate the stress-induced impairment of extinction. Decreased latency scores for the group injected with cycloheximide immediately after stress (RA/stress) compared with the increased scores of the stress group replicate retrograde amnesia for the stress memory. Longer latencies exhibited by the group that received a brief stress exposure (re-expose) demonstrate the recovery of the forgotten stress memory. The brief stress group showing less avoidance demonstrates that a 5-min restraint stress session before training was not sufficient to impair extinction retrieval.