Inactivation of hippocampus or caudate nucleus with lidocaine differentially affects expression of place and response learning.
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Involvement of the hippocampus and caudate nucleus in place and response learning was examined by functionally inactivating these brain regions bilaterally with infusions of lidocaine. Rats were trained to approach a consistently baited arm in a cross-maze from the same start box (four trials/day/14 total days). On Days 8 and 16 a single probe trial was given, in which rats were placed in the start box opposite that used in training and allowed to approach a maze arm. Three minutes prior to the probe trial, rats received bilateral injections of either saline or a 2% lidocaine solution (in order to produce neural inactivation) into either the dorsal hippocampus or dorsolateral caudate nucleus. On the probe trials, rats which entered the baited maze arm (i.e., approached the place where food was located during training) were designated place learners, and rats which entered the unbaited maze arm (i.e., made the same turning response as during training) were designated response learners. Saline-treated rats displayed place learning on the Day 8 probe trial and response learning on the Day 16 probe trial, indicating that with extended training there is a shift in learning mechanisms controlling behavior. Rats given lidocaine injections into the hippocampus showed no preference for place or response learning on the Day 8 probe trial, but displayed response learning on the Day 16 probe trial, indicating a blockade of place learning following inactivation of the hippocampus. Rats given lidocaine injections into the caudate nucleus displayed place learning on both the Day 8 and the Day 16 probe trials, indicating a blockade of response learning following inactivation of the caudate nucleus. The findings indicate: (1) the hippocampus and caudate nucleus selectively mediate expression of place and response learning, respectively (2), in a visually cued extramaze environment, hippocampal-dependent place learning is acquired faster than caudate-dependent response learning, and (3) when animals shift to caudate-dependent response learning with extended training, the hippocampal-based place representation remains intact.