Drugs vs Substrates?

If an substrate binds to an enzyme, what binds to a receptor?

When scientists learned that enzymes could change chemicals, they created a new term to specify the chemical that is changed. They called it a substrate — the chemical “acted upon.” The enzyme “binds” its substrate and an “active site” initiates the chemical change in the substrate. Later, scientists realized that there were non-enzymes that bound chemicals identically to enzymes, but that did not change their bound chemical. Instead, these non-enzymes “reacted” to the chemical in some way to initiate a biological process. The scientists then named these non-enzymes “receptors” to distinguish them from enzymes, but they failed to designate a new term for the substances-that-are-not-changed that fit these receptors. So scientists made do with “drug,” a misleading-at-best term by it’s accepted (and legal) definitions. In this article, we have used the word “drug” reluctantly — out of necessity. This is an unresolved problem with our technical language, not an attempt to imply any intrinsic pharmaceutical or therapeutic character to the receptor-drug interaction. ——SWF