INACTIVATION OF HIPPOCAMPUS OR CAUDATE-NUCLEUS WITH LIDOCAINE DIFFERENTIALLY AFFECTS EXPRESSION OF PLACE AND RESPONSE LEARNING

Involvement of the hippocampus and caudate nucleus in place and respon se learning was examined by functionally inactivating these brain regi ons bilaterally with infusions of lidocaine. Rats were trained to appr oach 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 tria l 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 pri or to the probe trial, rats received bilateral injections of either sa line or a 2% lidocaine solution (in order to produce neural inactivati on) into either the dorsal hippocampus or dorsolateral caudate nucleus . On the probe trials, rats which entered the baited maze arm (i.e., a pproached the place where food was located during training) were desig nated 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 th e Day 8 probe trial and response learning on the Day 16 probe trial, i ndicating that with extended training there is a shift in learning mec hanisms controlling behavior. Rats given lidocaine injections into the hippocampus showed no preference for place or response learning on th e Day 8 probe trial, but displayed response learning on the Day 16 pro be trial, indicating a blockade of place learning following inactivati on of the hippocampus. Rats given lidocaine injections into the caudat e nucleus displayed place learning on both the Day 8 and the Day 16 pr obe trials, indicating a blockade of response learning following inact ivation of the caudate nucleus. The findings indicate: (1) the hippoca mpus and caudate nucleus selectively mediate expression of place and r esponse learning, respectively (2), in a visually cued extramaze envir onment, hippocampal-dependent place learning is acquired faster than c audate-dependent response learning, and (3) when animals shift to caud ate-dependent response learning with extended training, the hippocampa l-based place representation remains intact. (C) 1996 Academic Press, Inc.