Influence of Gz and Gi2 transducer proteins in the affinity of opioid agonists to μ receptors

The affinity displayed by different opioids to μ receptors (ORs) was determined in mouse brain membranes incubated with antibodies directed to Gα subunits of the guanine nucleotide-binding proteins Gi2 and Gz. Assays were conducted with 10 pM 125I-Tyr27-β-endorphin in the presence of 300 nM N,N-diallyl-Tyr-(α-aminoisobutyric acid)2-Phe-Leu-OH (ICI-174,864), which prevented the binding of the iodinated neuropeptide to δ-ORs. Gpp(NH)p or the preincubation of mouse brain membranes with IgGs to G(i2)α or G(z)α subunits, promoted reductions in the affinity exhibited by the labelled probe. The potencies of β-endorphin, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and [D-Pen2,5]enkephalin (DPDPE) were reduced after impairing the coupling of μ-ORs to Gi2 or Gz proteins. Morphine showed a loss of affinity towards the μ-OR after preincubation of membranes with IgGs to G(z)α subunits. However, it retained its potency after treatment with the anti-G(i2)α IgGs. Conversely, [D-Ala2 D-Leu5]enkephalin (DADLE) and [D-Ser2, Leu5] enkephalin-Thr6 (DSLET) showed decreased affinity to μ-ORs after treatment with anti-G(i2)α IgGs, with no noticeable change following the use of IgGs to G(z)α subunits. The affinity exhibited by the opioid antagonists naloxone, naltrexone, naloxonazine and [Cys2,Tyr3,Om5,Pen7 amide]somatostatin analogue (CTOP) remained unchanged after either treatment. Therefore, the affinity exhibited by opioid agonists of μ-ORs, but not antagonists, depends on the nature of the G-protein coupled to these receptors.Peer Reviewe