Prostanoids are a subclass of eicosanoids consisting of the prostaglandins, the thromboxanes, and the prostacyclins. They are the cyclooxygenase metabolites of arachidonic acid, and include prostaglandin (PG) D2, PGE2, PGF2α, PGI2, and thromboxane A2. Their activities are related to a diversity of endogenous processes such as in?ammation, platelet aggregation, and vasoconstriction/relaxation. Prostanoid receptors belong to the subfamily of GPCR-A14, and can be grouped into three categories, based on the type of heterotrimeric G-protein activated by the different receptors, and thus the cellular response evoked (Gs (DP1, EP2), Gq (EP1), and Gi (EP3)).Prostaglandin D2 (PGD2) is an acidic lipid mediator that is derived from arachidonic acid by the sequential action of cyclooxygenase(s) (COX) and PGD2 synthase(s). Arachidonic acid is converted by COX-1 and COX-2 in a two-step process to first PGG2 and then PGH2. These unstable endoperoxide intermediates are converted to PGD2 by either the haematopoietic or lipocalin PGD2 synthase. PGD2 is produced in the brain where it might be involved in the regulation of sleep and other central nervous system (CNS) activities, including pain perception. In peripheral tissues, the richest cellular source of PGD2 is the mast cell.