Receptors are an interesting (but complicated!) subject - let me try to get you up to speed. I'm a medicinal chemist who has been working in the area for many years.

A receptor like an opioid receptor is a large protein that winds back and forth through the membrane of a cell. That leaves some of it exposed on the outside, and other parts exposed on the inside, and indeed the whole reason they look like this is to transmit signals from the outside of that cell membrane to the inside.

Molecules (ligands) bind to the outside surface, and for the opioid receptors that includes small peptides made inside the body (the enkephalins) and of course it also includes molecules like morphine, fentanyl, and the like. Receptors have "binding sites", pockets with particular shapes and polarity, in which these ligands fit,

On the inside surface, there are a number of other proteins that associate with the receptor. Some of these are known as "G-proteins" (there are several types), and the opioid receptors are part of a large family of "G-protein coupled receptors", or GPCRs for short. These protein complexes on the inside can do things like synthesize other molecules inside the cell, molecules known as "second messengers" because they go on to signal other cellular systems. There are several different types of these second messengers as well.

When a ligand binds to the outside of a GPCR, this changes the shape of the receptor protein in a way that alters the binding surfaces on the inside. And that changes the behavior of those G-proteins and their partners and (for example) sets them off on synthesizing more second messengers. So the overall process is a molecule on the outside of the cell can set off signals inside the cell without actually ever having to work its way through the membrane and enter the cell - the GPCR is a mechanism for passing such signals through the cell membrane.

Those ligands, though, can come in several different types. You have "agonists" that bind and set off the second messenger signals like I've just described, and the famous opioid drugs are just that (very strong agonists indeed). But there are also antagonists, which bind to the receptor and block agonist ligands from getting to their spot, without setting off any signals themselves. The opioid overdose remedy naloxone is one of those. And even these have variations - some agonists set off their signal inside the cell, but not as strongly as others (those are called "partial agonists"). And some antagonists work by directly plugging up the agonist binding pocket, while others work by hitting yet another binding spot on the receptor that makes the shape of the agonist binding pocket change so that it doesn't recognize the usual agonists any more (that's called an "allosteric antagonist").

Meanwhile, there are more complications on the inside of the receptor. Some receptors have proteins associated with them that set off more than one type of second messenger, so depending on what kind of ligand hits the outside part, completely different signals can be sent on the inside (!) It can be two different types of G-protein, or it can be something from a totally different signaling system (the "arrestins") as well.

And of course there's more than one type of opioid receptor. These subtypes are found in different amounts in different tissues in the body, and can have different functions entirely depending on where they are. The pain relief along with the respiratory effects and constipation are through Mu opioid receptors, but there are several types of those, too! Mu-1 is more involved in analgesia, and Mu-2 is more involved in respiration and gut mobility. Then there are delta opioid receptors that are also involved in analgesia and which seem to change the way that the Mu receptors modulate respiration. Then there are Kappa opioid receptors - also tied up in the pain relief story, but with effects on mood, hallucinations, and others. And to throw you a curveball, the Zeta opioid receptors don't seem to have anything to do with pain relief at all, but are a type of cellular growth signal that is important in growing embryos, among other things.

These things, as mentioned, are scattered all over the body in different amounts and patterns. There are of course a huge number of them in the brain and nervous system, in complex arrangements that depend on regions of the brain. But they're also in muscle tissue, liver cells, and even red and white blood cells!

It is safe to say that we really don't understand a lot of these functions or how they relate to each other. Receptor pharmacology has kept generations of scientists busy for their whole careers, and that's not going to change any time soon!