There are a number of neurotransmitters that may be involved in the development of major depression. Each neurotransmitter has a receptor, which receives the signal or information that each chemical messenger carries. The messengers are specific to each receptor, like a key (neurotransmitter) and a lock (receptor), and cause a number of reactions, both chemical and electrical, to take place within the receiving nerve cell. The neurotransmitters and receptors most discussed in major depression are norepinephrine and serotonin.