Figure 4.

(R)-SAL and (S)-SAL stereoisomers act as agonists on μ-opioid receptor. Functional assay of the μ-opioid receptor dose-response activation by (R)-SAL and (S)-SAL. Each ligand was assayed three times, each concentration, in duplicate using concentrations ranging from 1 × 10⁻⁸ M to 1 × 10⁻³ M. Values are expressed as mean cAMP levels (percentage relative de control) ± SEM. The luminescence is proportional to the intracellular cAMP levels (induced by forskolin); therefore, a decrease in cAMP levels signals the activation of the μ-opioid receptor, which is coupled to a Gi protein. EC₅₀ corresponds to the concentration of ligand eliciting 50% of the maximal response. (R)-SAL curve: degrees of freedom, 28 (two points excluded as outliers, at [Ligand](log, M) = −7, −7.5); R² = 0.8267. (S)-SAL curve: degrees of freedom, 28 (two points excluded as outliers, at [Ligand](log, M) = −7, −7.5); R² = 0.9072.