Science: The drug that gets to work on pleasure

IT IS ESTIMATED that up to 70 per cent of adults have clinical depression at some time in their lives. Depression can be triggered by bereavement, unemployment, divorce or separation, loneliness, after childbirth and even after viral infections.

Depressed people get tired easily, cannot concentrate, sleep badly and lose their appetite. Doctors therefore believed that stimulating the brain's natural activity might cure depression, and this is what all of the three main classes of anti-depressant drug attempt to do.

Monoamine oxidase inhibitors (MAOIs) were the first breakthrough in antidepressant treatment. MAOIs block the enzyme involved in the breakdown of the neurotransmitter noradrenaline - which means that they increase the brain's noradrenaline level. The logic is that in depression, there are decreased noradrenaline levels in the brainstem (a region involved in control of moods), so raising it should raise the patient's mood.

Although MAOIs work well they have some unpleasant side-effects, and can be fatal with some foods or alcohol.

The second wave of antidepressants were the tricyclic antidepressants (TCAs). They act in broadly the same way as MAOIs - to increase levels of noradrenaline - but do not have the same dangerous side effects. They can, however, take weeks to work. At present, they are the most widely prescribed antidepressant, as well as being used to relieve anxiety symptoms.

The newest type of antidepressant are the selective serotonin re-uptake inhibitors (SSRIs) - the best-known (some might say notorious) being Prozac. They do not act on the level of noradrenaline, but of serotonin (also called 5-HT) - sometimes known as the "pleasure chemical". The higher the levels in your brain, the happier you generally feel. A "re-uptake inhibitor" is a chemical which slows the rate at which our naturally-produced serotonin is reabsorbed within the brain.

Disturbances in serotonin transmission may underlie problems such as bulimia and kleptomania. Certainly, serotonin levels are lower than normal in depression.

For advice on depression contact MIND: Granta House, 15/19 Broadway, London, E15, 0181-519-2122; or the Samaritans.