Norepinephrine (NE) is the neurotransmitter often associated
with the "fight or flight" response to stress. Strongly linked to
physical responses and reactions, it can increase heart rate and
blood pressure as well as create a sense of panic and overwhelming
fear/dread. This neurotransmitter is similar to adrenaline and is
felt to set threshold levels to stimulation and arousal.
Emotionally, anxiety and depression are related to norepinephrine
levels in the brain, as this neurotransmitter seems to maintain the
balance between agitation and depression.