Dose responses of a panel of opioid ligands in HEK-DOR cells. (a) Dose dependent responses of opioid ligands obtained using DMR agonist assays. The maximal amplitudes were plotted as a function of agonist doses. (b) Dose-dependent desensitization by the DOR agonists of HEK-DOR cells to the repeated stimulation with DPDPE at 64 nM. (c) Dose-dependent inhibition by the DOR antagonists of HEK-DOR cells to the succeeding stimulation with DPDPE at 64 nM. For (b) and (c) the maximal amplitudes of the DPDPE DMR were plotted as a function of ligand doses, and data represents the mean ± s.d. for 2 independent measurements, each in duplicate (n = 4).