Figure 2
From: Analgesic tone conferred by constitutively active mu opioid receptors in mice lacking β-arrestin 2
Constitutive coupling of the μ-receptor with VDCC in β-arr2-/- but not β-arr2+/+ DRG neurons. A Constitutive coupling of GPCRs to VDCCs can be reversed by inverse agonists but not neutral antagonists. Accordingly the enhanced facilitation ratio in β-arr2-/- vs β-arr2+/+ neurons, shown in Figure 1 and reproduced here for comparison, was inhibited by naloxone (Nal), naltrexone (Ntx), but not by 6α-naloxol, 6β-naloxol (6α-Nal, 6β-Nal), 6β-naltrexol, (6β-Ntx), or by the combination of naloxone with 6α-naloxol (6α-Nal+Nal). These ligands had no effect in β-arr2+/+ neurons. The upper panels show exemplar traces generated by the two-step protocol (scale-bars represent 20 ms (horizontal) and 0.5 nA (vertical) and the data from 12-22 cells are summarized in the underlying graph, with P2/P1 ratio on the ordinate and treatment on the abscissa. ***p < 0.0001 vs β-arr2+/+, *p < 0.05 vs β-arr2+/+, #p < 0.05 vs untreated β-arr2-/-. The concentration of each drug in μM is indicated in parentheses. B. A single-step voltage protocol was used to measure Ba2+ current amplitude in the presence (2) or absence (1) of naloxone, or 6α- and 6β-naloxol, as shown by the exemplar traces in the top panels. Naloxone and naltrexone increased the current amplitude in β-arr2-/- but not β-arr2+/+ neurons, whereas 6α-, 6β-naloxol or 6β-naltrexol, or the combination of naloxone with 6α-naloxol, had no effect on the current amplitude in either cell type. The upper panels show exemplar traces for all conditions. Scale-bars represent 20 ms (horizontal) and 0.5 nA (vertical) and the data for 7-12 cells are summarized in the underlying graph, inhibition (%) shown on the ordinate and treatment on the abscissa. *p < 0.05 vs β-arr2+/+. The concentration of each drug in μM is indicated in parentheses.