Figure 2.

The action of racemic SAL is fully blocked by the μ-opioid receptor antagonist naltrexone. Levels of cAMP (percentage relative to control) detected after incubation with racemic (R/S)-SAL 150 μM in the presence of different concentrations of the antagonist naltrexone (1 × 10⁻⁵ M to 3 × 10⁻¹⁰ M) are shown. The activation of the μ-opioid receptor by the addition of the (R/S)-SAL 150 μM was assayed 30 min after the addition of the antagonist. This concentration of (R/S)-SAL elicits 80% of the maximal response of the system. The antagonism of the inhibitory action of SAL on μ-opioid receptor results in an increase of intracellular cAMP levels. The shown results are from one experiment performed in duplicate and each point represents one of the two replicates for each concentration of antagonist. Half-maximal inhibitory concentration (IC₅₀) corresponds to the concentration of antagonist reducing 50% the maximal response to the agonist. Degrees of freedom, 7; R² = 0.8550.