**1. Introduction**

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Hydrophobic ligands are able to penetrate inside the cell: they recognize intracellular receptors that migrate to the nucleus and regulate protein transcription. Hydrophilic ligands, in contrast, are unable to cross the plasma membrane: these extracellular ligands recognize transmembrane receptors that then produce intracellular messengers to affect the target cell function. Several families of transmembrane receptors are known: some are ligand-gated ion channels, and regulate the transmembrane voltage (depolarization or hyperpolarisation) or the intracellular Ca++ concentration; others are ligand-activated enzymes: some synthetize cGMP, others phosphorylate specific target proteins upon ligand recognition; and yet other receptors (known as "G Protein Coupled Receptors" or "GPCRs") activate intracellular trimeric G proteins in response to extracellular signals. These receptoractivated G proteins in turn activate enzymes responsible for "second messenger" synthesis (adenylate cyclase cAMP, or phospholipase C Inositol trisphosphate (Inositol(1,4,5)P3 or "IP3") and diacylglycerol), regulate ion channels, or activate other ("small") G proteins.
