Complex sentence: Synthetic opioid and opioid-derivative drugs activate these receptors (possibly by acting on the PAG directly, where these receptors are densely expressed) to produce analgesia, including heroin, morphine, pethidine, hydrocodone, oxycodone, and similar pain-modulating compounds.
Split into simpler sentences: Synthetic opioid and opioid-derivative drugs activate these receptors (possibly by acting on the PAG directly, where these receptors are densely expressed) to produce analgesia. These drugs include heroin, morphine, pethidine, hydrocodone, oxycodone, and similar pain-modulating compounds.