Manic depression is genetic and is caused by an excess of signalling cells in the brain, say scientists.

Research carried out in the US has found that people with manic depression or bipolar disorder have more of an important signal-sending brain cell than those without the condition.

The scientists, from the University of Michigan, say their discovery backs up previous theories that manic depression is a biological as opposed to a psychological condition.

They also suggest that their finding may explain why it appears to run in families.

Manic depression is marked by wild, cyclical mood swings. It usually starts when a person is in their late teens or early twenties and affects men and women equally.

These patients' brains are wired differently

Dr Jon-Kar Zubieta, University of Michigan

There are two forms of the condition. The first, type II, causes people to alternate between depression and hyperactivity.

The second, type I, is more severe and can cause frenzied and even psychotic episodes followed by deep depression.

Manic depression is currently treated with a range of mood-stabilising, anti-depressant or anti-psychotic drugs. It affects 1.5% of the population.

Dr Jon-Kar Zubieta, from the University of Michigan who carried out the research, said: "To put it simply, these patients' brains are wired differently, in a way that we might expect to predispose them to bouts of mania and depression.

"Now, we must expand and apply this knowledge to give them a treatment strategy based on solid science, not on the current method of trial and error.

"We should also work to find an exact genetic origin and to relate those genetic origins to what is happening in the brain."

Further research

He added the further research was needed to find out why more of these signal cells, called DTBZ, grow in people with manic depression.

"The reality is that we still only have sketches of what is going on in these brains, what the basic changes are and how they are related to the course of illness," said Dr Zubieta.

A spokeswoman for the UK mental health charity MIND said the research was inconclusive.

"Whilst this new study might help improve treatment for those people who have bipolar disorder it is not clear cut that if you do have these brain differences you will then go on to develop bipolar disorder.

She added: "We also need to be very careful that people are not discriminated against because of these neurochemical differences even though they may not develop bipolar disorder."

The study is published in the American Journal of Psychiatry.