SHIFT IN THE PERFORMANCE OF 24-MONTH-OLD WISTAR RATS IN THE MORRIS WATER ESCAPE TASK - A COMPARISON ACROSS 36 EXPERIMENTS

Spatial discrimination learning in aged rats serves as an animal model of cognitive aging. We assessed the replicability of spatial discrimi nation performance in the standard Morris water escape task. To this e nd the learning curves and the performance in a probe trial of 24-mont h-old outbred Wistar (HsdWin:Wu) control rats from 36 experiments were compared. These experiments had been performed at our laboratory unde r strictly controlled conditions over a period of 71 weeks. There was a very high variability in the learning curves between experiments. Th e initial performance level, i.e. the performance during the first ses sion; did not change systematically across the 36 experiments. In cont rast, the final performance level, i.e. the level reached in the fifth training session, decreased over the 71 week period, when the platfor m escape latency and the distance swum to reach the platform, measured as number of line crossings, were considered. In the last experiments of the series, learning curves were no longer seen: the rats did not improve their performance across the acquisition sessions. By contrast , the swimming speed and, in the probe trial, the bias for the quadran t where the platform had been positioned during training, did not chan ge. This indicates that a decrease across experiments occurred predomi nantly with respect to spatial orientation performance, whereas the mo tor performance appeared to be unchanged. Explanations for this observ ation, such as differences in viability between shipments and the poss ible occurrence of genetic drift, are discussed. (C) 1997 Elsevier Sci ence B.V.