If you remember anything at all about receptors for opiates (morphine) and opioids (drugs that act like morphine), remember the Mu (myoo) Receptor. It’s the main go-to receptor for pain relief in the body. There are also Kappa Receptors. They too are involved in pain relief, but to a lesser extent.
You will learn what you need to know about opioid analgesics when you study the drugs. But just in case you are curious about the receptor…
First of all, you don’t need morphine or any other drug to activate the Mu and Kappa receptors. They are naturally activated by three endogenous hormones: enkalphalins, endorphins, and dynorphins. Your body really needs this feature, because if you ever, say, cut off a fingertip while chopping onions, you want a little analgesia for the first few seconds before you actually look. The involvement of your psyche is another story for another time.
So morphine is the prototype, or typical example, of a Mu receptor agonist. We call this class of drugs opioid analgesics (morphine-related pain relievers). More on the specifics later. Some things you should be aware of regarding Mu receptors:
1) They are located in the central nervous system (mostly in the spinal column)
2) Their activation relieves pain, produces euphoria, and also DEPRESSES the CNS
3) CNS depression affects the control centers in the brain stem (remember those?). So it lowers your blood pressure and heart rate, but most important to remember is that is lowers your respiratory rate. It also may lower your level of consciousness.
4) The receptors love their agonists. So when an opioid attaches to a mu receptor over the course of time (chronic abuse or even chronic therapeutic use), the neurons they are attached to build more receptors and more mechanisms for processing the signals sent by them! This is part of a chain of events that causes the infamous opiate addiction.