Specialized cells in a living organism communicate with each other through extracellular molecules such as hormones, neurotransmitters, and growth factors. An immediate selection of the information offered is realized by cell-type specific expression of transmembrane receptors which transmit extracellular signals into the cell. Some membrane receptors for intercellular signal molecules are intramolecularly coupled to an effector system generating intracellular signals. Examples of this group of receptors are ligand-gated ion channels (eg, nicotinic acetylcholine, ionotropic glutamate, GABA A, and glycin receptors) and enzyme-linked receptors (eg, receptor tyrosin kinases and phosphatases, particulate guanylyl cyclases)[1]. In contrast, the majority of intercellular signals interacts with receptors representing one component of a three-protein transmembrane signaling system whose components interact sequentially and reversibly. Agonist binding to a specific receptor leads to an increased probability of interaction with one or more guanine nucleotide binding proteins (G-proteins), acting as transducers and signal amplifiers, which in turn modulate the activity of one or more effector systems (Fig. 1).