Flick the Switch on Fear

Scientists have discovered what happens in the brains of animals when they become frightened. The work, which has focused on a pair of almond-sized areas of the brain called the amygdala that function as the brain's fear centre, may also help scientists to develop novel therapies for human anxiety states such as post traumatic stress disorder.

Based at the Friedrich Miescher Institute for Biomedical Research in Switzerland, Cyril Herry and colleagues showed that when mice made the transition from being 'afraid' to 'not afraid' the behaviour was mirrored by a switch in activity level between two distinct groups of neurons found in an area known as the basal amygdala (BA). They measured the activity of cells in this area using tiny electrodes implanted into the rats' brains.

These BA neurons also link to other brain areas involved in processing incoming information about the animal's surroundings and situation. This allows them to pull together all this information to decide whether to go out searching for food, or to run and hide; in other words they act like a fear-on/fear-off switch, depending on which of the two groups of neurons is more active.

At the same time, a second group of researchers, this one led by Denis Paré from the Center for Molecular and Behavioral Neuroscience at the University of New Jersey, has identified a separate group of neurons in the amygdala that is also involved in the switch from being afraid to not afraid - by helping animals to 'unlearn' learned fear responses.

So when an animal is conditioned to be afraid of a neutral stimulus such as a light, by presenting it simultaneously alongside an unpleasant stimulus like an electric shock, the fear response to the light alone shows 'extinction' (i.e. it stops) after a few occasions of the light being presented without the nasty shock.

But Paré and his team showed that destroying the Intercalated (ITC) neurons in the amygdala of rats using a specific drug that targets and kills just these cells caused this extinction system to fail, suggesting that these ITC neurons are involved in the maintenance of learned fear.

Both groups suggest that the work could have implications for treatment of human fear and anxiety disorders including post traumatic stress disorder, which affects over a million people in the UK.