US scientists have identified a gene mutation that causes manic depression in as many as one in 10 patients.
Manic depression causes mood swings
The gene, GRK3, plays an important role in regulating the way the brain responds to chemicals such as dopamine which are known to control mood.
The mutation, discovered by a team from the University of California, San Diego, occurs in the gene's on-off switch.
This may mean that the gene is activated inappropriately, or turned off when it should be working.
Our hope is that discovery of genetic defects that cause bipolar disorder will lead to new drugs that can be directed to those specific genes
The researchers believe the mutation causes those who carry it to become hypersensitive to dopamine - leading to extremes of mood.
Manic depression, known technically as bipolar disorder, is a complex condition.
Patients veer between barely-controlled euphoria and feeling very down.
Drug therapies are available, but doctors estimate that they have little effect on up to half of patients.
Researcher Dr John Kelsoe said: "Our hope is that discovery of genetic defects that cause bipolar disorder will lead to new drugs that can be directed to those specific genes."
The findings are based on an analysis of DNA samples from more than 400 families.
Dr Kelsoe said: "We believe that a defect in GRK3 may make one super-sensitive to dopamine, somewhat like being born on cocaine."
Amanda Harris, of the UK charity Manic Depression Fellowship, told BBC News Online that better ways to diagnose manic depression and better treatments were badly needed.
She said it could take 10 years for the condition to be diagnosed.
"Manic depression probably has several underpinning reasons, and at the moment treatment is very hit and miss," she said.
The research is published in the journal Molecular Psychiatry.