Toronto, Nov 27 - Breathing in carbon dioxide is known to trigger panic attacks. Now scientists have unravelled why.

A new study by University of Iowa (UI) researchers shows that carbon dioxide increases brain acidity, which triggers a protein that is central to fear and anxiety behaviour. Inhalation of carbon dioxide in high does can also be lethal.

These findings opens the way for understanding the biological basis of panic and anxiety disorders and suggesting new lines of treatment.

Researchers focussed on a brain protein known as acid-sensing ion channel 1a (ASIC1a), which is abundant in the amygdala -- the region deep in the brain that processes fear signals and directs fear behaviour.

Blocking or removing ASIC1a is known to reduce innate fear and alters fear memory in mice.

"As long ago as 1918, scientists learned that carbon dioxide triggers abnormal responses in patients with anxiety disorders," said John Wemmie, associate professor of psychiatry and neurosurgery at the UI Carver College of Medicine, who led the study.

"But our study provides the first molecular evidence for a mechanism that explains how carbon dioxide can trigger fear and anxiety," he added.

"This is a new finding that the amygdala, which is considered the brain's computer processor for fear, can also function as a sensor for detecting chemical signals -- carbon dioxide and acidity, known to trigger panic attacks in susceptible individuals," Wemmie said.

The study suggests that evolution may have provided humans with a vital ability to detect and respond rapidly to carbon dioxide by placing within the same brain region the ability to detect the threat posed by carbon dioxide and the ability to initiate a "fight or flight" response.

Conversely, the study team, including study co-author Adam Ziemann, found that making brain tissue less acidic blunted fear behaviour produced by carbon dioxide and reduced learned fear, says an UI release.

"It's been suggested that controlling breathing with breath exercises could have anti-anxiety effects," Wemmie said.

These findings were published in the Nov 25 issue of Cell. (IANS)