Cholinergic receptor blockade in the rat impairs locale but not taxon strategies for place navigation in a swimming pool.

Abstract

Normal rats and rats drugged with atropine sulfate, a cholinergic muscarinic blocker, were evaluated in the Morris water task for their use of spatial navigation strategies. Atropine-treated rats were impaired on a place response of swimming to a platform hidden in a pool filled with opaque water. With extended training, they did learn the place response, though not with control precision, as shown by better than chance heading angles upon swim initiation, searches in the old location for a platform that had been moved, and by correct swims from novel starting locations. Acquisition could not be accounted for by habituation to the drug. In contrast with the acquisition deficit, pretrained rats were relatively unimpaired by the drug. In separate experiments, the strategies used by the drugged animals were examined. Atropine-treated rats were not impaired in acquisition or retention of a cue task, swimming to a visible platform, or a position response task, turning left to locate a platform, but both control and atropine-treated rats were unable to reverse position responses. Possibly, atropine-treated rats can use cue and position response strategies in part or in combination to acquire a place response. Atropine-treated rats were unable to acquire a place learning set, which involved swimming to a new location each day, a response that is eventually performed by normal rats in approximately one trial. They also were unable to perform a learning-set response, which they had acquired when undrugged. Insofar as the impairments following atropine are characteristic of a deficit in the use of a locale strategy, that is, making rapid use of relational properties of distal cues, whereas their successes are characteristic of the use of taxon strategies, that is, cue or position responses, the results suggest that locale systems of navigation are more importantly dependent on cholinergic brain mechanisms than taxon strategies. The resistance of preacquired place responses to atropine may suggest that normally rats acquire place responses by using a locale strategy but effect rehearsed responses by using taxon strategies. Finally, characteristic differences in acquisition, retention, and reversal of atropine-treated and control rats using different navigational strategies suggest new possibilities for interpreting various performance features displayed by animals after other types of treatment.

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