Memory-Enhancing Treatments Do Not Reverse the Impairment of Inhibitory Avoidance Retention Induced by NMDA Receptor Blockade

The aim of the present research was to verify whether the impairment of retention induced by the N-methyl-d-aspartate (NMDA) receptor blocker (+)-10,11-dihydro-5-methyl-5H-dibenzoa,d]cycloheptene-5,10 imine (MK-801) can be reversed by memory-enhancing treatments. Adult female Wistar rats were trained and tested in a step-down inhibitory avoidance task (0.3-mA foot shock, 24-h training-test interval). Animals were given an ip injection of saline (SAL) or MK-801 (0.0625 mg/kg) 30 minutes before training, and an ip injection of SAL, epinephrine (EPI) (25 microg/kg), the opioid receptor antagonist naloxone (NAL) (0.4 mg/kg), the glucocorticoid receptor agonist dexamethasone (DEX) (0.3 mg/kg), or glucose (GLU) (320 mg/kg) immediately after training. There was an impairment of inhibitory avoidance retention in the MK-801-SAL, MK-801-EPI, MK-801-NAL, MK-801-DEX, and MK-801-GLU groups. There was an enhancement of retention in the SAL-EPI, SAL-NAL, SAL-DEX, and SAL-GLU groups. A control experiment showed that the amnestic effects of MK-801 could not be attributed to decreased reactivity to the foot shock. The results suggest that memory-enhancing treatments directed at modulatory mechanisms do not reverse the memory impairment induced by NMDA receptor blockade.