Figure 2.

Methadone and fentanyl selectively activate mu-delta opioid receptor heteromers. Intracellular Ca²⁺ ion release mediated by increasing concentrations of either (a) fentanyl or (b) methadone, were measured in HEK-293 cells stably expressing opioid receptors and transiently transfected with the chimeric G-protein (Δ6-G_αqi4-myr). We have previously shown that morphine also selectively activates mu-delta opioid receptor heteromers (ref (29)). Response was measured as change in relative fluorescence units (ΔRFU = RFU_max – RFU_min). All points in the panels represent mean ± SEM from triplicate biological replications.