G-protein-coupled Receptors (GPCRs) represent a specific combination of surface receptors and G-protein-mediated signaling mechanism (G-protein = GTP/GDP-binding protein). Its essential function is to transmit a signal from the outside to the inside of a cell. GPCRs are the most numerous and most diverse type of surface receptors. Although each GPCR is unique and specific, all GPCRs function according to the same principle.

The receptor located on the surface of a cell is a 7-transmembrane receptor i.e. the protein-chain crosses the surface membrane 7 times thus forming a unique receptor structure (Figure 1). The signaling mechanism inside the cell is mediated by G-proteins (Figure 1). If the receptor is activated by a specific signal (Figure 1), the conformation of the receptor is changed. This conformational change transmits the extracellular signal to the inside of the cell and triggers the activation of the G-protein. The active G-protein has the potential to induce various cascades of intracellular signaling. GPCR activation can cause a broad spectrum of cellular responses: GPCRs are understood to be involved in every aspect of human physiology. To illustrate a few examples, GPCRs are involved in regulation of sleep, blood pressure, mood, food intake, perception of pain, control of cancer growth and immune response. Last but not least, one GPCR, rhodopsin, is responsible for detection of light and thus allows you in this very moment to read this text.