Spatial learning in rats is impaired after degeneration of the nigrostriatal dopaminergic system

We investigated spatial learning in rats with unilateral and bilateral lesions of the nigrostriatal dopaminergic system. We used the Morris water maze paradigm, which tests spatial forms of learning and memory and allows discrimination between sensory-motor and learning disabilities. Animals were trained preoperatively to learn the location of a spatially fixed hidden platform to escape from the swimming pool (acquisition training). A visual and a probe test were used before and after the acquisition training, respectively. Our results show that animals with unilateral lesions, although displaying longer escape latencies, have normal spatial memory abilities. Animals with bilateral lesions were able to swim as fast or even faster than animals with unilateral lesion. Despite the fact that these animals had learned the spatial navigation tasks preoperatively, bilateral dopaminergic lesions led to a profound deficit in ability to find a hidden platform during an acquisition task. In general, animals with bilateral lesions persisted in swimming along the pool walls and their spatial navigation performance during a probe test was very poor. These results suggest that deficit of the nigrostriatal dopaminergic system can affect the selection and maintenance of behavioral strategies in spatial navigation.