Opioid receptors. Coloured frontal Positron Emission Tomography (PET) scan showing the normal distribution of opioid receptors in the human brain. Opiod receptors are located on the exterior of nerve cells (neurons). When pain is detected by the body, endorphins are released and attach to the receptors. They temporarily prevent the nerve cells from firing and therefore provide pain relief. Opioid drugs mimic endorphins. By injecting a patient with an opioid tagged with carbon-11 (radioactive tracer), a colour-coded scan is produced, showing the concentration of opioid receptors from red (highest) through yellow and green to blue (lowest).