Effects of Corticosterone Injections After Training and Memory Reactivation on Recall of Fear Memory in Rats

Recent evidence indicated that when a stabilized memory is recalled or reactivated, it again becomes labile and initiates a time-dependent process referred to “reconsolidation”. Considering the documented role of stress hormones on emotional memory, the purpose of this study was to compare the effects of glucocorticoids on consolidation and reconsolidation of a fear conditioning memory. Adult male Wistar rats were trained in fear conditioning system. In experiment 1, rats were placed into context and after 180 s were given two 2 s, 0.4 mA shocks with an interval of 120 s. Twenty seconds after the final shock, rats removed from the context box and were injected with different doses of corticosterone or vehicle. In reconsolidation experiments, rats received 2 s, 0.4 mA shocks with an interval of 120 s (moderate memory) or given eight 2 s, 1.5 mA shocks with an interval of 62 s (strong memory). Thirty seconds after the final shock, rats removed from the context box. For reactivation, 24 h later rats were returned to the chamber for 90s. Immediately after reactivation, rats were injected with different doses of corticosterone or vehicle. Twenty-four hours after training or memory reactivation, rats were returned to the context box for 5 min. Seconds of freezing (defined as the absence of all visible movement oxcept respiration) during the retrieval testing were scored for each rat. The findings indicated that injections of corticosterone after training enhanced memory consolidation at dose of 3 mg/kg. Injections of the drug after memory reactivation did not change recall of moderate memory, but impaired recall of strong memory at dose of 3 mg/kg. The data indicate that glucocorticoids have opposite effects on consolidation and reconsolidation of contextual fear conditioning memory. Further studies are needed to determine the underlying mechanisms.