Depression - Role of Animals

Although the first antidepressants were discovered by chance and involved few if any animal experiments, much of what we now know about depression comes from research in animals.

Depressed rats do not seek food actively like their normal littermates.

We have learnt about the chemical imbalances in the brain that occur during depression mainly from examining animal brains. In people, such measurements can only be made by post-mortems and are somewhat unreliable because there is no way of controlling for factors such as lifestyle. Click here to view our video clip on why depression is difficult to study in people.

Much of future research into depression will also involve the use of animals - the brain is simply too complex to model well with computers or isolated cells in culture. However, modern technology means that the research may often start with human imaging and genetic studies. These should help to identify the precise brain areas altered in depression and the genes that are different in sufferers. With this information, researchers will be able to propose new theories to explain what causes depression and then test them in animals. For example, if researchers find a gene that is often altered in people with depression, they can make mice with the same mutation and see whether they have behaviour patterns like those of depressed people. Not only will this confirm that they have selected the right gene but it will also help design and develop drugs that will return these behaviours to normal.

No scientist is under the illusion that full depression itself can be modelled in animals. Depression is ultimately a disease of thought processes. The brain chemistry of rats and mice is very similar to ours, but a rodent is unlikely to become depressed in quite the same way as a human. Nevertheless, researchers can use animals to investigate the behavioural processes that are common across species and that become altered in depressed people. By understanding how these behaviours can be altered, clues may be found to what is causing depression. For example, researchers can expose rats or mice to mild but prolonged stress - changes in lighting conditions, or mate availability or food supply - and see how this affects the animal's interest in pleasurable events such as drinking sucrose solutions. Stress reduces the normal preference of rodents for sucrose over water - it is the equivalent of a depressed person's loss of pleasure in good food. Antidepressant drugs reinstate the preference for sucrose. Similarly, rats or mice put into a shallow pool of water, from which there is no exit, initially search for escape but after a while become immobile and float. Some antidepressants increase the time the animal actively seeks escape. The animals come to no lasting harm in either of these tests.

By combining a whole battery of these behavioural tests, together with neurochemical and other tests of an animal's response to stressful situations, scientists can investigate potential new antidepressants before trying them in people. In addition, by doing such tests, researchers can see whether their new theories of what causes depression are right. In this way, by going back and forth between people with depression and animals, scientists should eventually work out exactly what causes depression and how to deal effectively with this life-threatening and increasingly common illness.