Role of Efficacy as a Determinant of Locomotor Activation Induced by Mu Opioid Receptor Ligands in Male and Female Mice

Abstract

Background Ligand efficacy is a key determinant of drug effects mediated by receptor targets, and efficacy can be exploited to generate safer therapeutics. As one step in a broader project to examine efficacy requirements for different mu opioid receptors (MOR) effects, this study evaluated locomotor stimulation in mice produced by 8 MOR ligands that vary from low to high efficacy. Additionally, opioid agonist/antagonist mixtures provide a novel strategy to precisely calibrate levels of MOR activation, measure efficacy requirements for different MOR-mediated endpoints, and identify efficacy levels that might optimally balance effectiveness to produce therapeutic effects while minimizing side effects. Accordingly, effects of MOR ligands were compared to effects of fixed-proportion mixtures of the opioid agonist fentanyl and antagonist naltrexone. We hypothesized that MOR ligands would produce an efficacy-dependent increase in locomotor stimulation, and that fentanyl/naltrexone mixtures could be used to compare efficacy requirements for mouse locomotor stimulation relative to other in vivo and in vitro opioid effects. Methods We determined locomotor activity in adult male and female ICR mice after acute treatment with (in order of increasing efficacy) naltrexone, NAQ, nalbuphine, buprenorphine, hydrocodone, morphine, fentanyl, and methadone. Fentanyl/naltrexone mixtures were studied in fixed proportions of 1:1, 3.2:1, 10:1, 32:1, and 100:1. Dose-effect curves for each drug or mixture were determined in separate groups of 12 mice (6 male, 6 female). Treatments were administered subcutaneously 5 min before 100-min sessions, and locomotor activity was quantified as photobeam breaks in chambers with a floor area of 16.8 × 12.7 cm2. Data were analyzed by one- or two-way ANOVA, and the criterion for significance was p<0.05. Additionally, data with the mixtures were used to compare efficacy requirements for locomotor stimulation in this study relative to previously published data for other in vitro and in vivo effects. Results MOR ligands produced an efficacy-dependent increase in maximal locomotor stimulation. Similarly, for the fentanyl/naltrexone mixtures, increasing fentanyl proportions (pFentanyl) also produced graded increases in maximum stimulation (Emax). The curve relating pFentanyl to Emax was used to quantify the effective pFentanyl required to produce 50% of the fentanyl-alone Emax (EP50), and the EP50 (95%CL) served as a metric of the efficacy requirement for any opioid endpoint. In this case, the EP50 for locomotor stimulation in mice was high relative to EP50 values for other in vivo endpoints including fentanyl discrimination in rats and thermal antinociception in mice, rats, and rhesus monkeys, but lower than the EP50 for an in vitro assay of GTPɣS binding in mMOR-CHO cells. Conclusions These results confirm efficacy-dependent stimulation of locomotor activity by MOR ligands in mice. Additionally, these results illustrate the utility of fentanyl/naltrexone mixtures to precisely calibrate net MOR activation, quantify efficacy requirements across different opioid effects, and identify efficacy levels that might be sufficient to produce some drug effects but not others.