Scientists created new tools to study brain messenger proteins called neuropeptides in the brains of behaving animals, discovering they are the primary messengers in the fear circuit in mouse brains, and that multiple neuropeptides work together to achieve this--explaining why some clinical trials that target just one neuropeptide have failed. The new tools and findings can be used to direct drug development for fear-related neurological disorders like PTSD and anxiety, design better painkillers, and uncover new information for other neural circuits in the brain.