These receptors are present in several areas of the brain, namely the regions that manage perception of pain AND the regions that fire off feel-good sensations. Yup, the latter are the parts of your brain that tell you: "Whoa, you're high, buddy!" Opioid medications don't treat the source of the pain, though, which means they don't target muscular injuries, nerve distress, or bone disease. Instead, opioids funnel straight to the brain, the place where we process what the heck's going on elsewhere in the body, in a sense tricking the brain into not recognizing the source of discomfort. This is important.
But here's another thing that mu-opioid receptors do: control respiration and blood pressure. "Opiates hit many different receptors [in the brain], and different receptors respond in different ways," says psychiatrist Dr. John Tsuang, director of the Dual Diagnosis Treatment Program at Harbor-UCLA Medical Center. Some opioid receptors elicit euphoria by eliminating or dampening pain perception and giving users a baseline high. Triggering the others stops breathing. The nasty, unforgiving aspect of opioids, though, is that the receptors for euphoria develop tolerance, so that high feeling wears off, and the patient (or recreational user) needs more OxyContin than before to chase that initial high.