An overview on functional receptor autoradiography using [35S]GTPgammaS.

[35S]GTPgammaS binding autoradiography is a novel method to study the distribution and function of neurotransmitter receptors in tissue sections. This technique unifies the advantages of receptor-autoradiography and [35S]GTPgammaS binding, providing anatomical and functional information at the same time. Due to these two main features, it can also be called 'functional autoradiography'. [35S]GTPgammaS binding has long been used to study the first step of the intracellular signaling pathway, but until the mid 1990s it has only been performed on cell membrane extracts. Functional autoradiography evolved from this biochemical assay and ligand autoradiography, and is based on the increase in guanine nucleotide exchange at G-proteins upon agonist stimulation. With the technique, activation of G-protein-coupled receptors upon agonist binding can be detected, and, at the same time, the location of activated receptors can also be visualized. Thus only those presumably active G-protein-coupled receptors are visualized that can be involved in signal transduction. In the past 5 years the technique has become more and more frequently used in neuroscience, and it has been adapted to several receptors in different species, including also the human brain. [35S]GTPgammaS binding autoradiography can be used to describe the distribution of G-protein-coupled receptors. Some inferences on their coupling efficiency can also be drawn. Besides the localization of ligand binding sites, it provides information on the action of the ligand on the receptor: agonists, antagonists, and inverse agonists can clearly be distinguished. Moreover, [35S]GTPgammaS binding autoradiography can successfully be combined with other in vitro assays, like receptor autoradiography, in situ hybridization histochemistry, or even with biochemical and electrophysiological experiments. This review presents an overview on the history and the development of this technique. Its main advantages and limitations are summarized, together with a few basic technical questions. A number of experiments performed with [35S]GTPgammaS binding autoradiography so far, and some possible applications for the future, are also reviewed.