Fig. 3.

The third cytoplasmic loop of D2 like dopamine receptor (D2R) can confer G-protein coupled receptor kinase-independent and arrestin-dependent uncoupling to delta opioid receptor (DOR). (a) Schematic of DOR, D2R and the chimeric DOR receptor construct (DOR_D2R3rd-loop) in which the third cytoplasmic loop of DOR was substituted with the third cytoplasmic loop of D2R. (b) Average desensitization rates (± SEM) of the K_ir3 responses elicited by the DOR agonist, D-pen^2–5-encephelan (DPDPE), in oocytes injected with cRNA for the indicated receptor constructs (DOR or DOR_D2R3rd-loop) and K_ir3 channels (control, white bars). The desensitization rates are compared to those oocytes that were also injected in addition with cRNA for β-arrestin2 (ARRB2, black bars). Analysis of variances indicated significant differences in the desensitization rates between the different oocyte groups (p < 0.001). Tukey's post hoc test indicated that the average rate of desensitization in oocytes injected with cRNA for the DOR_D2R3rd-loop construct and ARRB2 was significantly different from the corresponding control (p < 0.001) as well as from oocytes injected with cRNA for DOR and ARRB2 (p < 0.05). The desensitization rate observed in oocytes injected with cRNA for DOR and ARRB2 was not significantly different from the corresponding control. (c) Dose-response curves for DOR or DOR_D2R3rd-loop-mediated K_ir3 activation produced with the DOR agonist, DPDPE. The response at each DPDPE dose is expressed as a percent of the average maximal response that was obtained (n = 3). The EC₅₀ (in nM) for DPDPE activation of DOR and DOR_D2R3rd-loop was 22.4 ± 8.8 and 28.0 ± 5.6 nM (mean ± SD), respectively, and were not significantly different between the two receptors.