Anticholinergic effects on acquisition of place learning in the Morris water task: spatial mapping deficit or inability to inhibit nonplace strategies?

The role of central cholinergic blockage in spatial learning was examined by testing atropine sulfate-treated (50 mg/kg) rats and saline-injected controls in the Morris water task using training procedures designed to promote the use of a spatial search strategy. First, constraints used in early trials deterred thigmotaxis. Second, an originally oversized hidden platform that nearly occupied the entire pool was effectively "shrunk" into the southwest quadrant of the pool by substituting smaller platforms over trials, a procedure intended to focus attention on the hidden platform in relation to extramaze cues. Task acquisition did not differ between groups, and on the probe trial both groups increased distance and latency and swam preferentially in the previously correct quadrant. Impairments caused by atropine sulfate may be the result of deficits in ability to inhibit nonefficient escape strategies.