Fig. 6.

EP₅₀ values as a metric of efficacy requirement for different effects produced by fentanyl/naltrexone mixtures. (A) Abscissa: Fentanyl proportion in different fentanyl/naltrexone mixtures. Ordinate: Maximum locomotor activating effects of each mixture expressed as a percentage of the fentanyl-alone maximum. Each point shows mean±S.E.M. for 12 mice, and nonlinear regression was used to calculate the EP₅₀, which is defined as the fentanyl proportion in a fentanyl/naltrexone mixture that would produce a maximum effect equal to 50% of the fentanyl-alone maximum effect. (B) EP₅₀ value (95% CL) for locomotor activation in the present study relative to EP₅₀ values for fentanyl/naltrexone mixtures determined in previous studies using various behavioral endpoints in mice, rats, and rhesus monkeys or in the in vitro assay of ligand-stimulated GTPɣS binding in CHO cells expressing cloned MOR. Assays with EP₅₀ values to the left of the shaded box have lower efficacy requirements than locomotor activation, whereas points to the right of the shaded box have higher efficacy requirements than locomotor activation. Abbreviations: Drug Discrim, drug discrimination; TW, warm-water tail-withdrawal with water temperature specified in °C.