WaterMaze Fact Sheet

Frequently Asked Questions about running Watermaze experiments

From Richard Morris (co-author of the WaterMaze program)

Testing mice in a watermaze. There are many who suspect that mice are not as good in a watermaze as rats, perhaps barely testable. They are certainly trickier to test, but with the right procedures, and given a bit of handling beforehand, as easily tested as rats and other rodents. Don’t give up if you’re first efforts don’t work.

Size of pool. A common (mis)conception is that the pool needs to be smaller. Not so. The issue is keeping the ratio of the surface area of the pool and platform around 50:1. In Edinburgh, we do this by training our mice in exactly the same 2 metre diameter pool as used to train rats, but we make the escape platform larger – typically 15 cm in diameter. We have several interchangeable “tops” for our platforms so that we can for example, use 20 cm diameter if we wish.

The logic of this is that the mice, no less than the rats, need to be able to see cues around the room to navigate and the further they move, the more these cues change. If you do make the pool smaller, you will unwittingly reduce the discriminability of extra-maze cues.

Depth of pool. The water does not need to be as deep in mouse training as with a rat, but if the pool is used for both rats and mice, you may not wish the height of side-walls to be much greater than 20-30 cm. We use 60 cm deep water, 30 cm between the water and the top of the side walls. However, for people testing mice only, the water depth need not be greater than 20 cm and the side wall height around 20 cm.

Water temperature. Most people train rats and mice at 25°C. Using a lower temperature than this can be needlessly stressful, a higher one can promote floating. Experience has taught that this is the best temperature to use.
We get to it with an automatic filling system, which is basically a cistern, completely plumbed into the pool. Filling is on a time-clock and via a thermostatically controlled valve which lets in an appropriate ratio of hot and cold water. We typically fill at around 27°C at 6 am. By the time we come into work, the temperature is about right. Manual override hot and cold taps enable us to fine tune this.

Draining the pool. Draining the pool regularly is essential for keeping it clean. We do this daily, but not all labs find this convenient. Again, we have a time-controlled drain that empties around 10 pm each night. In practice, we override this most times and wipe the internal side-walls clean. Still, if we are rushing to the pub after a hard day’s work…….we can let this go occasionally and still know that the pool will be filled with fresh water at the right temperature the next day.

Making the platform invisible. Many watermazes render the platform partially invisible (i.e. at least hard to see) with milk powder – the original way. Nowadays, most people use Latex liquid – widely used in the building industry to reduce the surface tension of concrete and so achieve a level surface. It is readily obtainable from builder’s supply houses and is not too expensive in bulk. About 250-500 ml per day suffices. Other suggestions include polystyrene beads that float on the water surface.

Color of the pool. The best mice to test are those with a pigmented retina that can actually see things and these tend to have dark coat colors (e.g. BL6 mice). Tracking these against a white water surface and a pool with white side-walls is best – even with state of the art Tracking Systems such as that provided in the Actimetrics/Watermaze Software computer system.
For this reason, we paint the inside of our pools, manufactured by farm-supply companies as drinking troughs for cattle, with two-part white paint used to paint the top-sides of boats. International two-part paint, for example, is a very good resin-based system that can later be scrubbed clean hundreds of times without flaking. If you choose to buy this, you’ll find it is also good for painting boats!