system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we have opioid receptors, nicotonic receptors, cannabinoid receptors, etc? These phrasings imply that our brains are almost meant to accept these molecules. How right or wrong is that, or do we even know?
gpt
The body produces chemicals that act on these receptors to transmit a signal or perform some function. For example, the nicotinic AcetylCholine Receptor that you mention. A nerve may be carrying a signal. At the end of the nerve the signal causes the release of AcetylCholine into the synapse. The Acetylcholine binds to the nicotinic receptor on another nerve and the signal continues on in the second nerve or some other action occurs.

In nature there are many chemicals and many have structural similarities. Some will act upon a receptor in the body. That does not imply that the receptor was meant to accept that chemical. It is just chance because of the great similarities between chemicals in nature that a receptor that is triggered by a chemical produced in the body is also triggered by a chemical produced in a plant.