The human body naturally produces its own opiate-like substances and uses them as neurotransmitters.
These substances include endorphins, enkephalins, and dynorphin, often collectively known as endogenous opioids.
Endogenous opioids modulate our reactions to painful stimuli. They also regulate vital functions such as hunger and thirst and are involved in mood control, immune response, and other processes.
The reason that opiates such as heroin and morphine affect us so powerfully is that these exogenous substances bind to the same receptors as our endogenous opioids.
There are three kinds of receptors widely distributed throughout the brain: mu, delta, and kappa receptors