The brain circuits underlying defensive behaviours, such as freezing or flight, in mice are reported in a paper published this week in Nature. The study suggests that highly organized neuronal circuits, which are critical for higher brain function, may also be involved in evolutionarily old systems that promote survival in the face of danger.
Fear is an evolutionarily conserved state that triggers various forms of defensive behaviours. The periaqueductal grey region (PAG) has been identified as an important part of the brain involved in this response to threat. However, the specific pathways into and out of the PAG that facilitate defensive behaviours have remained poorly understood.
Andreas Luthi and colleagues identify a pathway from the amygdala to the PAG that projects to premotor neurons to induce freezing in male mice. In one experiment, the authors exposed mice to a remote-controlled toy snake and determined that activation of neurons in the ventrolateral PAG is necessary to generate freezing. They also find that this ‘freezing’ pathway interacts with circuits that mediate flight. Together, these findings uncover the neural circuitry involved in the rapid selection of appropriate defensive behaviours, which enables mice to adapt to changing threat levels or contextual challenges.
This knowledge could improve our understanding of the inappropriate fear responses that have been implicated in human anxiety disorders.