A research team conducted by scientists at the California Institute of Technology (Caltech) enables a clear understanding of how the brain reacts to fear.
David J. Anderson, the Benzer Professor of Biology at Caltech and a Howard Hughes Medical Institute investigator-describe a microcircuit in the amygdala that controls, or "gates," the outflow of fear from that region of the brain.
The microcircuit contains two subtypes of neurons that are antagonistic-have opposing functions-and that control the level of fear output from the amygdala by acting like a seesaw.
"Imagine that one end of a seesaw is weighted and normally sits on a garden hose, preventing water-in this analogy, the fear impulse-from flowing through it," said Anderson.
"When a signal that triggers a fear response arrives, it presses down on the opposite end of the seesaw, lifting the first end off the hose and allowing fear, like water, to flow."
The study may someday provide a new target for developing more specific drugs for treating fear-based psychiatric illnesses like post-traumatic stress disorder, phobias, or anxiety disorders.
The team found that a gene that encodes an enzyme known as protein kinase C-delta - which is expressed in about half the neurons within a subdivision of the amygdala's central nucleus, the part of the amygdala that controls fear output.
They were able to fluorescently tag neurons in which the protein kinase is expressed; this allowed the researchers to map the connections of these neurons, as well as to monitor and manipulate their electrical activity.
The cells that decreased their activity in the face of fear-inducing stimuli clearly corresponded to the kinase C-delta plus neurons Anderson's lab had isolated, while those that increased their activity corresponded to the kinase C-delta minus neurons.
"While these studies shed light on only a small part of the picture, they are an important step in that direction," Anderson said.
The study is published in this week's issue of the journal Nature.