Table 2 Efficacy and potency of opioid agonists for the stimulation of [³⁵S] GTPγS binding in HEK 293 cells stably expressing hMOR or Ala^348,353 hMOR. Agonist efficacy was calculated as the maximal difference between specific [³⁵S] GTPγS binding in the presence and absence of agonist and is expressed in fmol/mg. EC₅₀ values were obtained from curve fitting of dose response curves shown in Figure 3. Receptor densities were 4.13 ± 0.26 and 1.5 ± 0.20 pmol/mg membrane protein for hMOR and Ala^348,353 hMOR, respectively. Statistical analysis was performed using Student's t test: the asterisks (**p < 0.01, ***p < 0,001) refer to values significantly different from corresponding values for hMOR (Student's t test,)

From: Enhanced spontaneous activity of the mu opioid receptor by cysteine mutations: characterization of a tool for inverse agonist screening.