Figure. Concentration-dependent activation of OPRD1 by leu-enkephalin. Reporter cells were transfected with the expression plasmid for opi­oid receptor delta 1 (OPRD1) and treated with various concentrations of leu-enkephalin. Data points shown are the mean ± SEM of an experiment performed in triplicate, and the curve is a fit to Hill equation with an EC50 of 6.1 nM.