Spatial learning deficit in dopamine D(1) receptor knockout mice.

Dopamine D(1) receptors are expressed in the hippocampus and prefrontal cortex, suggesting a role in cognition. Dopamine D(1) receptor-deficient mice (D(1)-/-) were used to investigate the role of this receptor in spatial learning and memory. Using the Morris water maze, mice were trained to locate a hidden platform. Subsequently, the platform was removed from the maze and mice were scored for the percentage of time spent in the target quadrant and the number of crossings through the target position. D(1)-/- mice had significantly longer escape latencies compared to wild-type (D(1)+/+) and heterozygous (D(1)+/-) littermates and showed absence of spatial bias during the probe trials. In a visually cued task, D(1)-/- mice performed better than on the hidden platform trials, but maintained slightly higher escape latencies than D(1)+/+ and D(1)+/- mice. Naive D(1)-/- mice exposed only to the cued task eventually acquired identical escape latencies as the D(1)+/+ and D(1)+/- mice. Sensorimotor reflexes, locomotor activity, spontaneous alternation and contextual learning were not different among the groups. These results indicate that D(1)-/- mice have a deficit in spatial learning without visual or motor impairment, suggesting that dopamine D(1) receptors are involved in at least one form of the cognitive processes.