A binding question: the evolution of the receptor concept

In present-day pharmacology and medicine, it is usually taken for granted that cells contain a host of highly specific receptors. These are defined as proteins on or within the cell that bind with specificity to particular drugs, chemical messenger substances or hormones and mediate their effects on the body. However, it is only relatively recently that the notion of drug-specific receptors has become widely accepted, with considerable doubts being expressed about their existence as late as the 1960s. When did the receptor concept emerge, how did it evolve and why did it take so long to become established?