A family of proteinase-activated receptors that are specific for thrombin. They are found primarily on PLATELETS and on endothelial cells. Activation of thrombin receptors occurs through the proteolytic action of thrombin, which cleaves the N-terminal peptide from the receptor to reveal a new N-terminal peptide that is a cryptic ligand for the receptor. The receptors signal through heterotrimeric gtp-binding proteins. Small synthetic peptides that contain the unmasked N-terminal peptide sequence can also activate the receptor in the absence of proteolytic activity.