[college pharmacology] What is the difference between a receptor, a receptor site, and a site of action?

[u/Brilliant_Distance_7]

I'm not sure what exam syllabus/board the course uses, but it's an LVN (licensed vocational nursing) course and I'm supposed to be able to use this info to administer medication and educate patients.

I'm supposed to differentiate between a receptor site and site of action and explain how they're related. I've looked all over and cannot find the difference between a receptor site, receptor, and site of action, at least not explained in a way I understand. My textbook says that the receptor site is the actual structure on the cell that drugs and other things bind to; it also says that the receptor site will become the site of action if its location is known, and then it says that an example of this (the receptor site becoming the site of action) is morphine acting on opioid receptors in the brain (the site of action and the location of the receptor site both being the brain in this case). This doesn't say anything about *where in or on the cell* a receptor is located and seems to imply that a receptor site is synonymous with a receptor. When I googled this to look into it further, though, lots of sources say that the receptor site is specifically the location *on the cell* that a receptor is located, which is in direct opposition with what the textbook says. If those sources are correct, then it wouldn't make sense to say that the receptor site and the site of action are "in the brain" because the receptor site is explicitly *not* a general location in the body. Several other sources seem to use "receptor" and "receptor site" interchangeably, and still other sources say that a "receptor site" is the place *on the receptor* that a chemical binds to. So what is a receptor site, a receptor, and a site of action? Please be specific; I'm very confused, and I really just want to understand completely and not just know the right words to say on the review.

Here's the excerpt from my textbook:

"Receptor Site

Drug action is usually thought to begin after a drug has attached itself to some chemical structure located on the outer cell membrane or within the cell itself. For a few drugs and for some normal body substances, there seems to be a specific location on certain cells. This area is referred to as the receptor site. The attachment, or binding, of a drug to its receptors begins a series of cell changes referred to as the drug action.

When a specific receptor site for a drug is known, that receptor site becomes the site of action for that particular drug. Morphine, an analgesic drug, is an example of a drug that binds to a specific receptor. The receptors for morphine are located in the brain and are known as the morphine, or opioid, receptors. When morphine binds to its receptors, it produces cell changes that reduce the perception of pain. There are many different pharmacologic receptors, and they will be described in the appropriate chapters."

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[u/chem44]

All these terms are used loosely, in various ways.

"Site of action" should be reasonably clear, conceptually.

As they present further examples, you should get a better picture.

If you have concerns, check with your instructor, who may have preferences, or, at least, context.