Answer: ENDOGENOUS
Explanation: Receptors readily binds with biochemical messangers (called ligand) in the body system.
A particular receptor will only bind with a specific ligand, which translates that a specific receptors which readily binds with opiate drugs (such as morphine and oxycodone) for reducing body response to pain, must have been in the past, had an interaction with an endogenous biomolecule similar in molecular composition with that of opioid pain relievers .
