A similar effect in humans might explain how the drug has helped millions of people world-wide to cope with depression.

One in five Britons are likely to suffer from depression during their lifetime, yet medical opinion is divided as to its cause.

Some believe it is a genetic disease or a chemical imbalance, while others claim it is triggered by environmental factors.

Treatments for the disease include counselling and in extreme cases electric shock therapy.

But it is Prozac, now taken by more than 37 million people world-wide, which has been hailed as revolutionising sufferers' lives.

Prozac is one of a group of drugs known as selective seratonin re-uptake inhibitor (SSRIs), which make the mood-enhancing brain chemical, or neurotransmitter, serotonin more available in the brain.

However, the drug takes weeks to have an effect, and scientists do not know the exact mechanism by which it helps to combat depression.

Brain cell birth

One in five Britons will suffer from depression during their life

New Scientist magazine reports that researchers at Princeton University in America, set out to examine the impact of Prozac on the brain.

They knew that depressed people have a smaller than usual hippocampus, a structure involved learning and memory.

They also knew that chronic stress can reduce the birth of brain cells, known as neurogenisis, in the brains of rodents - and that stress is a contributing factor in depression.

The Princeton researchers found that activating a particular receptor for serotonin in rats' brains increased the birth of brain cells. So they decided to see if SSRIs like Prozac would have the same effect.

They found that five rats injected with Prozac for 21days had 69% more new brain cells than another injected with saline.

The researchers believe that the rise and fall of brain cell birth may be an important factor in explaining why people become depressed and respond to SSRIs.

Chief researcher Professor Barry Jacobs, director of Princeton's neuroscience programme, said it could explain why Prozac takes time to improve mood.

"The time needed for these newly generated cells to mature and make appropriate connections provides an explanation for the 'therapeutic lag' in antidepressant therapy," he said.

Cary Cooper, professor of psychology at UMIST, welcomed the research but said it had to be linked to social factors.

"People usually become depressed after a severe life event - they are unable to form a relationship, they lose a significant job, suffer bereavement, or have a bullying boss.

"We have seen in human and animal studies that persistent chronic stress damages the immune system, suppressing T-cell growth. So it is quite possible that it also affects cellular growth in the brain."

"What we need to know is whether traumatic events make people depressed via brain cell death. If they do, we could then identify which events are most damaging and take measures to avoid them."