Two genes that influence the activity of nerve cells in the brain may play a key role in a person's risk for bipolar disorder, marked by dramatic swings from depression to manic behaviour.

Two genes that influence the activity of nerve cells in the brain may play a key role in a person's risk for bipolar disorder, marked by dramatic swings from depression to manic behaviour.

The findings are not expected to lead to a genetic test for the risk of the condition but could help show how it arises and lead to better treatments, researchers say.

An international team of scientists examined the genomes of 10,596 people mainly from Britain and the US, including 4387 with bipolar disorder, or manic depression.

The researchers found those with bipolar disorder more likely to have certain variants of the ANK3 and CACNA1C genes. Proteins made by the genes help govern the flow of sodium and calcium in the brain, influencing the activity of nerve cells.

"The key importance of this is that it gives us a clear idea of the sorts of chemicals and mechanisms in the brain that are involved in bipolar disorder," said Nick Craddock of Cardiff University, who helped lead the study. "That will help researchers to develop better approaches to diagnosis and treatment."

Because bipolar disorder tends to run in families, scientists have been trying to pinpoint genes involved.

This was the largest genetic analysis of its kind on the disease, which affects an estimated 1% to 3% of adults worldwide, Mr Craddock said.

The brain disorder causes extreme shifts in mood, energy and ability to function. It is marked by high periods of elation or irritability and low periods of sadness and hopelessness that can last months.

The US government's National Institutes of Health helped fund the research, reported in the journal Nature Genetics. Dr Thomas Insel, director of the NIH's National Institute of Mental Health, said: "It's not going to tell us the whole story but it's something to build on.".