G-protein coupled receptors (GPCRs) are a diverse super-family of proteins located within the plasma membrane of eukaryotic cells which have a common architecture consisting of seven-transmembrane (7-TM) segments, connected by extracellular (ECL) and intracellular (ICL) loops. They differ from other 7-TM proteins in their ability to activate guanine-nucleotide binding proteins or β-arrestin and so initiate a signaling cascade. Therefore, they are among the most important pharmaceutical drug targets. GPCRs are activated by a wide variety of stimulants, including light, odorant molecules, peptide and non-peptide neurotransmitters, hormones, growth factors and lipids, and control a wide variety of physiological processes including sensory transduction, cell–cell communication, neuronal transmission, and hormonal signaling. Upon activation by a stimulant (binding or conformational change of ligand), the conformation of the receptor is altered, which can result into two principal signal transduction pathways involving the G protein-coupled receptors: the cAMP signal pathway and the phosphatidylinositol signal pathway.Analysis of the human genome revealed at least 799 unique GPCRs. One widely adopted scheme classify the GPCRs into six families, with the most important and extensively studied being: family A (Rhodopsin-like); family B (Secretin); family C (Metabotropic glutamate/pheromone); family D (Fungal mating hormone); family E (Cyclic AMP); family F (Frizzled/Smoothened).