The effect of acute restraint stress and dexamethasone on retrieval of long-term memory in rats: an interaction with opiate system

This study investigated whether application of acute restraint stress or dexamethasone, as a glucocorticoid receptor agonist, impaired retrieval of long-term memory and if pretreatment with opiate antagonist naloxone blocked their effects on memory retrieval. Young adult male rats were trained in one trial inhibitory avoidance task (1 mA, 1.5 s foot shock). On retention test given 48 hr after training, the latency to re-enter dark compartment of the apparatus was recorded. Thirty min before retention test, the rats were exposed to 10 min of restraint stress in a Plexiglas tube or were injected with dexamethasone (1 mg/kg) with or without prior treatment of naloxone (1 or 2 mg/kg). The results showed that both acute restraint stress and dexamethasone impaired retention performance, and their effects were blocked by naloxone pretreatment. The applied stress increased circulating corticosterone levels as assessed immediately after the retention test, indicating that stress–induced impairment of memory retrieval mediates, in part, by increasing plasma levels of glucocorticoid. These findings further indicate that acute restraint stress and glucocorticoids impair retrieval of long-term memory, and provide evidence for the existence of an interaction between glucocorticoids and opiate system on this process.