Adrenaline, or epinephrine, is a stress hormone secreted from the adrenal glands on the kidneys. It plays a major role in preparing the body for a fight-or-flight reaction in threatening environments. An adrenaline rush is a sudden increase in the secretion of adrenaline from the adrenal glands. This happens when the brain communicates to the glands that there will be a need for a fight-or-flight response. The cause of an adrenaline rush need not be an actual physical threat but can also be an imagined threat, strenuous exercise, heart failure, chronic stress, anxiety or a disorder of the brain or adrenal glands.
Stress Hormones and Memory

While adrenal glands constitute a major site for adrenaline synthesis, adrenergic neurons in the brain stem also produce adrenaline. These neurons contain the enzyme PNMT, which is required for a gland or neuron to convert noradrenaline into adrenaline. Stressful situations accelerate the activity of adrenergic and noradrenergic neurons. This can have a profound, negative effect on memory, according to a research team in the October 2008 Journal of Neuroscience. When stress chemicals function as neurotransmitters, they affect the storage of memories by activating the amygdala, a center of the brain involved in the processing and storage of negative emotions. Normally, people remember things better if they are replayed many times in the mind, but a single emotionally significant event may suffice for neurons to generate long-lasting networks.