Basic neurotransmitters influence whether or not an action potential will be produced in the post-synaptic cells they affect. Glutamine, an excitatory neurotransmitter, increases the likelihood of an action potential, whereas GABA, an inhibitory neurotransmitter, decreases the likelihood. Beyond these basic neuotransmitters, however, there are several neurotransmitters that have modulatory effects. By acting on post-synaptic membranes, they can dampen or intensify the effects of the basic neurotransmitters. Because their effects are diffuse rather than concentrated in a small part of the brain, they have widespread effects on mood, motivation, memory, and other aspects of behavior. Therefore, many drugs that are used to treat disturbances in behavior work by interacting with these modulatory neurotransmitters.

Depression, which affects many people at some time in their lives, is treated with three different classes of drugs, which are classified according to their molecular structure and function. One group is the MAO inhibitors. Monoamine...