I know the answer is no. But what then explains the name of these receptors being specific to Cannabinoid found in cannabis? Aren't Cannabinoid receptors exclusive to Cannabinoid? Why are they named like that, if not?
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7$\begingroup$ This is a case where the receptors were discovered while researching cannabinoids, but before they had discovered the endogenous chemicals that bind to the receptors. Once those were found, they were named the endocannabinoids. $\endgroup$– user137Commented Aug 17, 2018 at 7:54
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2$\begingroup$ You also have ammonia receptors. Are we also meant to smell it? $\endgroup$– gfdsalCommented Oct 12, 2020 at 17:25
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3 Answers
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Receptors
Any drug or compound with specific effects has a receptor. You can read about this general concept in Goodman and Gillman's The Pharmacological Basis of Therapeutics. Chapter 1 introduces the concept, chapters 3 and 5 expand further. As far as the history of this concept is concerned, receptors mediating the specificity of action was first clearly articulated by Paul Ehrlich (in the context of dyes and toxins), and further developed by John Langley (and Ehrlich again in response). Cannabis and cannabinoids are not unique in this regard. Illicit and therapeutic drugs of all kinds have a target binding sites (receptors). In almost all cases, these receptors are bound by endogenous compounds as well.
Does having a receptor mean we should consume something?
There was a case to be made that having a great number of receptors that bind psychoactive compounds in cannabis means there is a physiologic role for some cannabinoid (especially given the pattern of expression) and suggested the existence of an endogenous cannabinoid before it was discovered. This was discussed in the nature paper that first identified the receptor and its functional expression and in the science paper that reported the discovery of endocannabinoids. Generally, it doesn't suggest that we are "meant to consume cannabis". Toxins and poisons also have receptors, and we certainly don't want to consume those.
As far as the name is concerned, the comment by @user137 is correct. Receptors are sometimes named by exogenous drugs that activate them, in particular when these compounds are discovered and studied before an endogenous ligand is found. Opioid receptors are another example.
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$\begingroup$ So I think I had the misconception that cannabinoid receptors allow only cannabinoid to bind with it. Can you confirm that multiple ligands can bind to a single substrate? (So there's no exclusive lock and key mechanism) $\endgroup$– yathishCommented Aug 18, 2018 at 6:53
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$\begingroup$ @yathish that's correct. Cannabinoid is an entire class of compounds. Many different specific cannabinoids bind cannabinoid receptors, with varying affinities. The lock and key model is useful, but not entirely accurate. For most (possibly all) receptors, more than one ligand can bind. $\endgroup$– De NovoCommented Aug 18, 2018 at 17:47
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In addition to De Novo's excellent answer, I would like to add that sometimes, the fact that we have receptors for something is precisely a sign that we should not consume that thing.
When it comes to taste and smell in particular, many receptors are evolutionarily conserved precisely because their physiological role is to warn us of a danger. For example, diamines such as putrescine and cadaverine smell well, terrible, as their names suggest. And we are very good at detecting these smells, because these molecules are produced during organic matter putrefaction; in other words, we have receptors (recently identified) designed specifically to tell us that certain foods are not suitable for consumption.
The same principle applies to olfactory receptors for volatile molecules harboring sulfhydryl (-SH) groups, that often smell like rotten eggs or feces - for the same reason. Mercaptan is even used to give natural gas its distinctive smell (it's initially odorless), making sure that we can detect any gas leak that could cause an accident - because we have really good receptors for it.
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Cannabinoid receptors are a part of the endocannabinoid system, which encompasses native to our body ligands such as Anandamide, 2-Arachidonoylglycerol, Palmitoylethanolamide, and a few others.
The reason they are named after the cannabis plant is because of the role cannabinoids played in identifying and characterizing those receptors, a pattern that you'll see in biochemistry a lot, e.g. AMPAr and NMDAr receptors named because AMPA/NMDA are their selective ligands, even though both are glutamatergic receptors, nicotinic/muscarinic acetylcholine receptors, or CART(Cocaine- and Amphetamine-Regulated Transcript) named for the fact that cocaine and amphetamine regulate its transcription levels.
For what it's worth, not all cannabinoid system proteins are named after cannabis, such as the more recently characterized NAGly, GPR55, and GPR119 receptors.
