Effects of NMDA receptor channel blockers, MK-801 and memantine, on locomotor activity and tolerance to delay of reward in Wistar-Kyoto and spontaneously hypertensive rats.

N-Methyl-D-aspartate (NMDA) receptor blockade enhances motor activity and stimulates dopamine metabolism, effects shared with classical psychostimulant drugs. The present study aimed to characterize behavioral effects of two NMDA receptor channel blockers, MK-801 and memantine, in both Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In Experiment 1, SHR rats demonstrated higher spontaneous locomotor activity and spent more time in the central area of the open field apparatus compared with WKY rats. Rats of both strains pre-treated with MK-801 (0.01-0.3 mg/kg) or memantine (1-32 mg/kg) demonstrated dose-dependent increases in the total distance traveled and time spent in the central area. Experiment 2 was based on the two-lever discrete-trial delayed reinforcement task in which rats could press one lever to obtain one pellet immediately or another lever for four pellets delivered after a variable delay (0-60 s). Tolerance to delay of reward did not differ between strains. MK-801 (0.03-0.3 mg/kg) and memantine (1-10 mg/kg) produced small, but significant, facilitation of the large-reward lever responding and markedly impaired operant performance at higher dose levels (increased number of missed trials). For both experiments, effects of MK-801 and memantine were more pronounced in WKY compared with SHR rats. Additional studies are needed to address the utility of noncompetitive NMDA receptor blockers in the treatment of attention deficit and hyperactivity disorder.