Table 1

Potencies and efficacies of various ligands for causing a loss or increase in surface μ opioid receptor number

DrugnEC50Surface Receptor Staining% Maximal Internalization
nM % of control
Etorphine48.7  ± 1.747  ± 8100
Morphine4ND91  ± 217  ± 21-a
Fentanyl4113  ± 3065  ± 766  ± 41-a
RTI-1a43.3  ± 1.549  ± 799  ± 6
RTI-1b47.8  ± 2.248  ± 899  ± 3
RTI-1c10NA103  ± 4NA
RTI-1d10NA108  ± 31-b NA
Buprenorphine9NA110  ± 31-b NA
Naloxone15NA116  ± 21-b NA

Change in surface receptor staining was determined as described inExperimental Procedures. The percentage of maximal internalization was calculated to be the amount of internalization observed relative to that caused by etorphine in each experiment. Results for morphine, RTI-1c, RTI-1d, buprenorphine, and naloxone were based on a 1-h treatment with 10 μM of each drug. Curves were fitted to a standard four-parametric logistic equation with SigmaPlot (SPSS Inc., Chicago, IL). Values are means ± S.E.

  • ND, not determined; NA, not applicable.

  • 1-a Significantly less than etorphine-induced internalization,P < .05 by Student's t test.

  • 1-b Significantly more than control surface staining,P < .05 by Student's t test.