By altering the genes of rodents, US scientists have shown that blocking a key enzyme in the brain reduces anxiety.

It also avoids the sedating effects often associated with anti-anxiety drugs, says a team at the University of California at San Francisco.

The researchers hope to develop new treatments for anxiety, which affects an estimated 183 million people worldwide.

Other gene-altered rodents include this 'super-intelligent' mouse.

The mice lack the gene for an enzyme known as protein kinase C epsilon (PKC).

It is present in many neurons of the brain, but its function is unclear.

Lab studies showed that mice without the enzyme showed reduced anxiety but stayed alert. They also had lower levels of the stress hormone corticosterone.

The anti-anxiety effect seems to be mediated by a different mechanism from conventional tranquilisers such as Valium.

'Mental distress'

About 25% of people develop an anxiety disorder at some time in their lives. These include phobias, panic attacks and post-traumatic stress disorders.

Symptoms include restlessness, fatigue, difficulty concentrating, irritability, muscle tension, and sleep disturbances.

The UK mental health charity Mind gave a cautious welcome to the research.

"We welcome anything that looks likely to alleviate mental distress however humans are much more complicated than mice," a spokesperson told BBC News Online.

"Blocking an enzyme is not going to cause problems that provoke anxiety - like relationship problems or stress at work - to go away."

Scientists have not yet found an anti-anxiety drug that is not addictive or without side-effects, the spokesperson added.

The rodent research is published in The Journal of Clinical Investigation.