PAR1, PAR3, and PAR4 are activated by thrombin. There are species-specific differences in thrombin receptor expression in platelets and other cell types, in which differences in thrombin concentrations may considerably affect platelet activation of distinct PARs. As seen in human platelets, PAR1 and PAR4 are the functional thrombin receptors, whereas PAR3 and PAR4 are functional thrombin receptors in mouse platelets
Thrombin receptors are also differentially expressed in cell types, e.g. PAR1 is expressed in fibroblasts, smooth muscle cells, sensory neurons and glial cells, whereas the other two are less clearly defined.
There are various roles depending on location of activation. Fibroblasts and smooth muscle cells induces growth factor and matrix production, migration and proliferation. Sensory neurons induces proliferation and release of neuroactive agents.