How Fight or Flight Actually Gets Triggered (its not the chemical you think):

I got interested in the other functions of bone. It turns out, in addition to being more "alive" than we think, bone provides a pathway for the secretion of certain hormones. Bone also plays an important part in the triggering of the acute stress response known as the fight or flight response. The hormone osteocalcin helps bring about this acute stress response. When the body is under acute stress, the brain signals the skeleton to release this osteocalcin.
The reason why bone specifcally triggers this response is pretty interesting. As the skeleton evolved to protect us, the researchers of the study I looked into hypothesize that the stimulation of flight or fight response makes perfect sense as yet another physiological mechanism that protects the organism.
 "If you think of bone as something that evolved to protect the organism from danger -- the skull protects the brain from trauma, the skeleton allows vertebrates to escape predators, and even the bones in the ear alert us to approaching danger -- the hormonal functions of osteocalcin begin to make sense" is how they phrase it.
In order to test weather osteocalcin was necessary or sufficient to the triggering of fight or flight, the researchers took mice that had been engineered to not produce osteocalcin and wild type mice and presented them with stressors. The mice that had been engineered, as it turns out, were completely indifferent to the stressors while the wild type mice were stressed as predicted. Additionally, the researchers were able to create a fight or flight stress response in the modified mice just by injecting a large amount of osteocalcin into the mice.
This study shows how animals that do not have adrenal glads and cannot produce adrenaline can have normal fight or flight responses. Adrenaline is not necessary for fight or flight.
In conclusion, adrenaline gets a lot of credit for the fight or fight response, when that credit should really go to the skeleton and the hormone osteocalcin.