 The ("R")-enantiomer of modafinil, known as armodafinil, was also subsequently found to act as a D2High receptor partial agonist, with a Ki of 16 nM, an intrinsic activity of 48%, and an EC50 of 120 nM, in rat striatal tissue.The ("S")-enantiomer is inactive with respect to the D2 receptor.Modafinil has been found to directly inhibit the firing of midbrain dopaminergic neurons in the ventral tegmental area and substantia nigra of rats via activation of D2 receptors.