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Shallow water (paddling) variants of water maze tests in mice

Robert Deacon

Abstract

When Richard Morris invented his water maze in 19847, most behavioural work was done in rats. However, the greater understanding of mouse genetics led to the mouse becoming increasingly important. But researchers found that mutant mice were prone to problems like passively floating or diving when they were tested in the Morris water maze13. This was hardly surprising considering their natural habitat; rats swim naturally (classically, the “sewer rat”), whereas mice evolved in the dry areas of central Asia. To overcome these problems, I considered whether shallow water would be a sufficient stimulus to provide escape motivation for mice. This would also avoid the problems of drying the small creatures with a towel and then putting them in a heated recovery chamber to avoid hypothermia, which is a much more serious problem than with rats; the large ratio of surface area to volume of a mouse makes it particularly vulnerable to rapid heat loss. Another consideration was whether a more natural escape strategy could be used, to facilitate learning. Since animals that fall into water and swim away from the safety of the shore are unlikely to pass on their genes, animals have evolved a natural tendency to swim to the edge of a body of water. The Morris water maze, however, requires them to swim to a hidden platform towards the centre of the maze – exactly opposite to their evolved behaviour. Therefore the paddling maze should incorporate escape to the edge of the apparatus. This feature, coupled with the use of relatively non-aversive shallow water, embodies the “Refinement” aspect of the “3 Rs” of Russell and Burch9. We experimented with various types of maze design; the common feature was that the water was always shallow (2 cm deep) and escape was via a tube piercing the transparent wall of the apparatus. Other tubes (“false exits” were also placed around the walls but these were blocked off. From the inside of the maze all false exits and the single true exit looked the same. Currently we are using a dodecagonal (12-sided) maze with 12 false/true exits set in the corners. In a recent development we have used a transparent paddling Y-maze successfully.
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