Cholinergic receptor blockade impairs spatial localization by use of distal cues in the rat

Spatial localization was studied in the Morris water maze. The task required rats to escape from cool water (made opaque by milk) by finding a submerged, invisible platform located at a fixed place within the room. The start point was varied randomly, and there was no local cues to indicate the position of the platform. After training, the platform was moved. Rats subjected to central cholinergic receptor blockade with atropine sulfate were compared with normal rats and rats receiving peripheral cholinergic blockade with atropine methylnitrate. The controls for the use of search strategies, as compared with spatial mapping, were a group of blind rats and a group of rats for which the platform was moved from trial to trial. Measures of escape latency, swim distance, initial heading error, posttrial rearing on the platform, and search strategy after platform displacement revealed that the control rats and the atropine methylnitrate rats used a spatial mapping strategy to locate the platform. The atropine sulfate treated rats adopted a search strategy like that of the blind rats and the rats for which the platform was randomly moved: Their escape latency and swimming distance decreased across trials, including reversal trials, but their initial heading errors remained unchanged. The results support the idea that central cholinergic systems are important for spatial mapping, which demands the use of distal visual cues, but not for spatial localization, which requires other search strategies and possibly the use of proximal tactile, kinesthetic, and visual cues. Consistent with this idea, certain features of the atropine sulfate treated rats' behavior also suggested a novel explantation for some aspects of atropine stereotypies.