Place learning strategy of substantia nigra pars compacta-lesioned rats

The substantia nigra pars compacta (SNc) and the dorsal striatum are often considered to be necessary for stimulus-response (S-R) habit learning, whereas the dorsal hippocampus is considered to be necessary for relational (declarative) memory. Spatial learning is a kind of relational learning that occurs when a rat is released from different locations (variable start) in a water maze to find a submerged platform that is kept in a constant location. However, when the rat is always released from the same starting position (constant start), it can learn to find the platform oriented by a fixed configuration of cues, that is, by S-R learning. To test the critical role of the SNc in S-R and relational learning, the authors tested adult male Wistar rats, sham-operated or with a lesion in the SNc, in these 2 versions of the water maze task. The SNc lesion was induced by bilateral intranigral infusion of 0.5 micromol 1-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine. Although the SNc-lesioned rats learned the variable-start version as effectively as sham rats did, they were significantly impaired in learning the constant-start version of the task.