Although a number of receptor classes exist in humans, by far the most abundant and therapeutically relevant is represented by the G protein-coupled receptor (GPCR or GPCRs) class.  It is estimated that there are some 100,000 genes within the human genome, and of these, approximately 2% or 2,000 genes, are estimated to code for GPCRs.  Receptors, including GPCRs, for which the endogenous ligand has been identified are referred to as “known” receptors, while receptors for which the endogenous ligand has not been identified are referred to as “orphan” receptors.  GPCRs represent an important area for the development of pharmaceutical products: from approximately 20 of the 100 known GPCRs, 60% of all prescription pharmaceuticals have been developed.  This distinction is not merely semantic, particularly in the case of GPCRs.  Thus, the orphan GPCRs are to the pharmaceutical industry what gold was to California in the late 19th century—an opportunity to drive growth, expansion, enhancement and development. 
GPCRs share a common structural motif.  All these receptors have seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane (each span is identified by number, i.e., transmembrane-1 (TM-1), transmebrane-2 (TM-2), etc.).  The transmembrane helices are joined by strands of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane (these are referred to as “extracellular” regions 1, 2 and 3 (EC-1, EC-2 and EC-3), respectively).  The transmembrane helices are also joined by strands of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane (these are referred to as “intracellular” regions 1, 2 and 3 (IC-1, IC-2 and IC-3), respectively).  The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell. 
Generally, when an endogenous ligand binds with the receptor (often referred to as “activation” of the receptor), there is a change in the conformation of the intracellular region that allows for coupling between the intracellular region and an intracellular “G-protein.”  It has been reported that GPCRs are “promiscuous” with respect to G proteins, i.e., that a GPCR can interact with more than one G protein.  See, Kenakin, T., 43 Life Sciences 1095 (1988).  Although other G proteins exist, currently, Gq, Gs, Gi, and Go are G proteins that have been identified.  Endogenous ligand-activated GPCR coupling with the G-protein begins a signaling cascade process (referred to as “signal transduction”).  Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition.  It is thought that the IC-3 loop as well as the carboxy terminus of the receptor interact with the G protein. 
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different conformations: an “inactive” state and an “active” state.  A receptor in an inactive state is unable to link to the intracellular signaling transduction pathway to produce a biological response.  Changing the receptor conformation to the active state allows linkage to the transduction pathway (via the G-protein) and produces a biological response.  A receptor may be stabilized in an active state by an endogenous ligand or a compound such as a drug.