Radioligand Binding Assays: Theory and Practice

Abstract

Transmitters and hormones generate signals in target cells by interacting with a signal transducing protein or receptor. Receptors may be located in the cytosol, allowing for example, steroid and thyroid hormones to control gene transcription. However, the largest family comprises G-protein linked seven transmembrane spanning receptors located in the plasma membrane. In addition to the endogenous transmitter or hormone, selective drugs are continuously being developed that act via receptors or discrete recognition sites. Drugs may mimic the action of the endogenous transmitter by binding to a receptor to induce a biological response such as muscle contraction and therefore function as an agonist. At the other end of the spectrum, competitive antagonists bind to receptors without inducing a functional response themselves but block the action of an agonist or endogenous transmitter. It is difficult to differentiate between agonists and antagonists in a binding assay and both classes of compounds will therefore be referred to as ligands.