Fig. 2.

Effects of fixed-dose naltrexone pretreatments to fentanyl and fixed-proportion fentanyl/naltrexone mixtures in an assay of thermal nociception in male rhesus monkeys. Left panels show effects of fentanyl alone and after increasing naltrexone doses administered as a 15-minute pretreatment to fentanyl at 50°C (top) and 54°C (bottom) thermal intensities. Right panels show effects of fentanyl alone, naltrexone alone, and three fentanyl/naltrexone mixtures at 50°C (top) and 54°C (bottom). Abscissae: cumulative intramuscular fentanyl dose (left panels) or cumulative drug dose (right panels) in milligrams per kilogram. Note that for data with fentanyl/naltrexone mixtures in the right panels, the abscissa shows the fentanyl dose in the mixture, and the naltrexone dose = fentanyl dose × naltrexone proportion. Ordinates: %MPE. All points represent mean ± S.E.M. value of four monkeys.