Displacement

Displacement is the laboratory phenomenon at the core of radioligand binding assays: when a competing compound is introduced to a receptor preparation already occupied by a labelled reference molecule, it can dislodge some of that label from the binding site. The fraction of label remaining across a range of competitor concentrations generates the binding curve from which affinity values — chiefly the Ki — are calculated.

The technique isolates a single variable: how strongly two molecules compete for the same site on a purified or membrane-bound target. It says nothing about what the winner does once bound. An agonist and an antagonist can produce identical displacement curves if their affinities happen to be equal.

How it works · its role

In a typical assay, a receptor preparation — usually disrupted cell membranes presenting the target protein — is incubated with a radioligand: a known molecule tagged with a radioactive isotope, most often tritium (³H) or iodine-125 (¹²⁵I). At equilibrium, the radioligand occupies a predictable proportion of available binding sites.

The test compound is then added across a concentration range. As its concentration rises, it outcompetes the radioligand for the same sites, reducing how much radioactivity stays bound. The concentration that produces 50% displacement is reported as the IC₅₀.

Because the IC₅₀ depends partly on how much radioligand is present in the assay, it is converted to the Ki (inhibition constant) using the Cheng-Prusoff equation — a correction that yields an affinity value independent of assay conditions. A lower Ki means higher affinity: fewer molecules are needed to compete effectively at that site.

Relevance to substances & effects

When a substance page lists Ki values — for example, 3 nM at the μ-opioid — those numbers come from displacement assays. They describe how tightly the substance competes for that receptor relative to a standard radioligand.

High affinity at a receptor does not imply any particular effect. MDMA, serotonergic psychedelics, and many antipsychotics all displace radioligands from the 5-HT₂A receptor, yet they behave very differently once bound: some activate it, some block it, some partially do both. Functional assays — which measure downstream signalling rather than binding competition — are needed to resolve that distinction.

Binding tables on Psychedex are therefore a starting point, not a conclusion. They show which receptors a substance can reach, and how competitively. The subjective effects those interactions produce require converging evidence from functional studies, animal models, and human data.