Many of the most significant advances in neurobiology in the 1970s relate to the study of receptor function Receptors are proteinaceous membrane components that, when occupied by a specific ligand (neurotransmitter, neuromodulator, or hormone), will initiate a cellular response The most important techniques that have been developed recently to advance such studies involve ways of measuring directly the interactions between a neurotransmitter or drug and its receptor. With few exceptions, these procedures involve an in vitro technique in which animals are sacrificed, their brains removed and dissected into various specific regions according to some standardized procedure, and the dissected regions homogenized and centrifuged to yield a membrane preparation that includes the receptors of interest. Sometimes crude synaptosomal (P₂) pellets are used in the final binding assay that follows these procedures, but more often homogenates are the source of receptor material. Aliquots of the homogenate are then incubated with various concentrations of a radioactive ligand, specific for the receptors of interest, in the presence or the absence of displacing concentrations of a “cold” (nonradioactive) ligand, often called a displacer