Figure 3.

Responses of cells expressing the μ opioid receptor. (A–C). Immunofluorescence localization. Cells stably expressing a FLAG-tagged μ-opioid receptor were incubated for 1 hr in the absence of ligand (A) or in the presence of 100 nM etorphine (B) or 100 nM morphine (C), then fixed in 4% formaldehyde. The receptor was detected by incubation with the monoclonal antibody M2, followed by fluorescein isothiocyanate-conjugated anti-mouse antibody (16). Specimens were examined by confocal microscopy. Scale bar = 10 μm. (D) Chemotaxis, assessed as in Fig. 1, in response to morphine or etorphine. Similar results were obtained in three separate experiments. Values represent the mean ± SE of six determinations. (E) Inhibition of forskolin-stimulated cAMP accumulation was measured in response to increasing concentrations of morphine or etorphine, as in Fig. 2B. Similar results were obtained in three separate experiments. Bars = mean ± SE of three determinations.