Fig. 5.

Relationship between MOR agonist and fentanyl/naltrexone mixture effects on in vitro activation of GTPɣS binding and in vivo locomotor stimulation. Abscissa: E_max for each drug or mixture to stimulate GTPɣS binding in CHO cells expressing cloned MOR from previously published studies (see text for citations). Data are expressed as a percentage of the maximum effect of the high-efficacy MOR agonist DAMGO, which was included as a standard in each study. Ordinate: E_max for each drug or mixture to stimulate locomotor activity in the present study. Data are expressed as a percentage of the maximum effect produced by fentanyl. The blue dotted line shows linear regression for single-molecule opioids on the linear portion of the curve (morphine, fentanyl, and methadone excluded). The gray solid line shows linear regression for the fentanyl/naltrexone mixtures. Abbreviations: BUP, buprenorphine; HYD, hydrocodone; METHD, methadone; MORPH, morphine; .