By the mid-1960s, it had become apparent from pharmacologic studies that opiate drugs were likely to exert their actions at specific receptor sites, and that there were likely to be multiple such sites.
Sredinom 1960-tih, postalo je očevidno iz farmakoloških studija da opijatni lekovi deluju na specifičnim receptorskim mestima, i da je verovatno da postoji više takvih mesta.