Peptides. The largest and most complex neurotransmitters are the peptides (or neuropeptides), such as oxytocin and endorphin. They are composed of chains of 2 to 39 amino acids. Most of the dozens of types of neurotransmitters in our brain and peripheral nervous system are peptides, but their concentrations are much lower than that of the amino acids and monoamines (which are also known as the classical neurotransmitters). Many neurons that distribute a classical neurotransmitter also distribute a peptide that increases or decreases the postsynaptic neuron's receptivity of the neuron's primary transmitter—thus modulating its effect.

In addition to moving within neural networks, peptides use our circulatory system and air passages to move throughout our body/brain to modulate our broad range of pleasure and pain. A peptide action that occurs simultaneously in a large number of related cells can powerfully affect the decisions we make within the continuum of emotionally charged approaching and retreating behaviors, such as to drink-urinate, agree-disagree, buy-sell, and marry-divorce. In effect, the shifts in the body/brain levels of these molecules allocate our emotional energy: what to do, when to do it, and how much energy to expend on the activity.

A peptide's message can vary in different body/brain areas, just as a two-by-four can be used in many different ways in the construction of a house. For example, angiotensin is a peptide that activates the seeking and conserving behaviors that regulate our body's fluid levels. In our brain it does this principally by activating feelings of thirst and the consequent behaviors that seek water. In our body it causes kidneys to conserve water. The situation is similar with many drugs. For example, alcohol can excite or sedate, depending on the amount ingested and the drinker's emotional state. Endorphins can similarly reduce intense pain and increase euphoria.

A lot of chattering is continuously occurring within our brain—all mediated by awesomely complex but elegant simple chemical verbs and adverbs!