Dose response curves of representative ligands on δ opioid receptors Varying concentrations of opioid ligands were used to determine the potency and efficacy of each ligand in inhibiting the effect of 5 μM forskolin in producing cAMP, as described under methods. Maximal cAMP levels were in the range of 400–1000 pmole/well. Data presented are the average data from 2 or more experiments carried out in duplicate. Data have been normalized as described under methods. Error bars represent standard error of the mean of the normalized data. (■) Etorphine, (□) Lofentanil, (▼) SKF 10047 and (●) Fentanyl.