EFFECTS OF MK-801 ON LEARNING AND MEMORY AS ASSESSED USING A NOVEL WATER MAZE

The effects of the NMDA receptor antagonist MK-801 {(+)-10,11-dihydro-5-methyl-5H-dibenzo [a,d]-cyclohepten-5,10 imine hydrogen maleate} on learning and memory were assessed using a water maze. The maze was a traditional type of maze with alleys and choices between various paths, but set inside a pool of water to a height of 25 cm. Different mazes could be configured by altering the arrangement of open vs. closed doors. Both the time required to reach an out-of-the-water exit platform and the errors made during the swim from start to finish were recorded. Learning was assessed during the first 10 to 20 trials in a new maze configuration, while memory was tested after the maze was well learned. Three experiments, some with several phases, were performed. These experiments compared the effects of 0.1 mg/kg of either (+)-MK-801, or (-)-MK-801 vs. saline on learning new maze configurations as well as swimming well-learned mazes. Neither of the MK-801 isomers impaired performance of a previously learned maze. (+)-MK-801 clearly slowed learning of new mazes as measured by both maze completion time and errors committed, while (-)-MK-801 had a significant but smaller effect on learning. Rats given (+)- or (-)-MK-801 (0.1 mg/kg) for 16 days while learning one maze and then challenged to learn a new maze without drug administration performed no differently on the new maze than controls, suggesting that the acute effect of MK-801 on learning is not long lasting.