How do you react to a frightening situation? Do you fearfully freeze? Do you run for cover? Or do you stand your ground? All animals – including us – tend to react to stress in one of these three ways, and researchers from Stanford University have just identified a “switch” in the brain that makes mice do the latter, essentially turning off fear. The finding could help treat people with anxiety-related disorders.
From owls to snakes, mice are faced with many predators. When they spot an attacker, they usually either freeze or hide, but a small proportion will courageously stand their ground, flicking their tails with aggression.
The researchers identified two cell clusters in the brain that control these behaviors – one stimulates the freeze response and the other stimulates the aggression response. They exposed mice to a widening shadow, representative of a bird of prey closing in, and monitored the activity in their brains.
The team concluded that a structure in the brain called the ventral midline thalamus (vMT) was related to the threat responses. The vMT is connected to two other parts of the brain called the basolateral amygdala and the medial prefrontal cortex, which have previously been liked to anxiety, by two different nerve tracts. The researchers discovered that stimulating each nerve tract produced different responses to the predator.
They found that stimulating a cluster of cells that trigger the nerve connected to the basolateral amygdala made the mice much more likely to freeze in response to the looming predator. Meanwhile, when the researchers stimulated the cells associated with the nerve connected to the medial prefrontal cortex, the mice were much more likely to react with aggression – a behavior seldom seen naturally in mice exposed to a predator.
These mice stood their ground, angrily rattling their tails to signal their aggression. “It’s the mouse equivalent of slapping and beating your chest and saying, ‘OK, let’s fight’,” said Andrew Huberman, senior author of the paper published in Nature.
Although this research has only been conducted on mice, the researchers are hopeful that it could pave the way to treating people with conditions such as excessive anxiety and post-traumatic stress disorder (PTSD) in future.
As the researchers repeated their experiment, they found that the mice became more habituated to the predator, and their vMT stress response began to diminish. The team posits that in people with phobias, constant anxiety, or PTSD, this does not occur, and our equivalent to the vMT keeps firing. Discovering how to manipulate the vMT could really help people who suffer from these conditions by literally reducing the fear they feel.