Brain's fear centre has built-in suffocation sensor

Washington, Nov 26 (ANI): A new study in mice has revealed that the portion of the brain responsible for registering fear and even panic is equipped with a built-in chemical sensor that is triggered by a primordial terror - suffocation.

The research team from University of Iowa hopes that their new discovery may help to explain and perhaps even correct what goes wrong in those who suffer from panic attacks.

"The amygdala has been thought of as part of the fear circuitry of the brain. Now we see it isn't just part of a circuit, it is also a sensor," said John Wemmie of the University of Iowa, Iowa City.

Amygdala is a structure that stimulates the sympathetic nervous system for fight-or-flight and links to other brain regions involved in the response to threat. The amygdala is also known to play a role in both innate and learned fears.

It is a well known fact that carbon dioxide inhalation can trigger panic attacks, and that patients with panic disorder are particularly susceptible.

The new study showed that the amygdala not only senses the threat posed by carbon dioxide, but it also initiates a response.

It was previously shown that the acid-sensing ion channel-1a (ASIC1a) is particularly abundant in the amygdala and other fear circuit structures, where it is required for normal responses in tests of fear behaviour. ASICs are sensitive to pH and become activated when pH levels fall.

"It's interesting that evolution positioned an acid sensor right in this central circuit. Detecting an elevated carbon dioxide is critical for survival. When you are suffocating, this circuit triggers mechanisms for escape or relief of the problem," said Michael Welsh from University of Iowa.

"Because oxygen-breathing organisms are under a constant threat of asphyxiation, it could be argued that the threat of suffocation has had a primary influence on shaping the brain's defensive systems," said Stephen Marin of the University of Michigan, Ann Arbor.

She added: "The present discovery that chemosensors in the amygdala are involved in generating fear responses to a variety of aversive stimuli suggests that a system that evolved to generate behavior to defend against suffocation was subsequently adapted to deal with both innate and learned threats in the external environment

The contribution of both the amygdala and ASIC1a to fear behaviour led researchers to suspect that a reduced pH might induce fear behaviour by activating the channels.

They found that inhaled carbon dioxide reduced brain pH and evoked fear behaviour in mice. Mice breathing 5pct carbon dioxide tended to avoid open spaces more than usual and, in standard tests of fear learning conducted in the presence of 10pct carbon dioxide, the mice displayed exaggerated freezing behaviours.

However, animals lacking those acid-sensing ion channels showed less fear, a condition that was reversed when the channels were reinstated specifically in their amygdala.

The study appears in the journal Cell, a Cell Press publication. (ANI)