Narcotics mimic the action of the body's naturally occurring opiate-like substances called endorphins, enkephalins, and dynorphin. There are three types of opioid receptors present throughout the brain: mu receptors, delta receptors, and kappa receptors. Narcotics have most of their effect on the mu receptors, of which there are seven subtypes. The body's natural opioids attach to one or more of these receptor types to regulate hunger, thirst, mood, immune response, and awareness of painful stimuli. In the brain stem, the opioid receptors are active in regulating blood pressure, alertness, and breathing.