As part of a project to behaviorally characterize the eNOS knockout (that is the endothelial nitric oxide synthase deficient mouse), we compared spatial learning using two paradigms with different reinforcement contingencies. These were the Morris type water maze, where escape from an aversive state provides a negative reinforcement, and the Olton type radial maze, where food acquisition provides positive reinforcement. Surprisingly, in the water maze the eNOS-/- mice demonstrated superior performance as compared to C57BL/6 and eNOS+/+ wild type littermates, whereas in the radial maze the eNOS-/- mice exhibited deficits in performance as compared to controls. The eNOS-/- mice also showed higher levels of anxiety as measured by the elevated plus maze and the open field test of anxiety. These results suggest that the motivational demands of the spatial learning tasks can determine whether a mutation will show an improvement or a deficit in performance. Although it is possible that other factors could account for these seemingly opposite results, it is likely that they are due to different reinforcement contingencies inherent in the tasks. For one, these results indicate that an assessment of spatial learning cannot rely on the employment of either the water maze or radial maze alone. An obvious conclusion is that at least both must be part of a strategy for the phenotyping of spatial learning in mutant mice.

Supported by BMFZ.

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