Modification of opioid agonist binding by pertussis toxin

Membrane fractions prepared from rat striate, cortex and midbrain were treated with pertussis toxin, which has been shown to adenosine diphosphate (ADP)-ribosylate the GTP-binding protein G_i, reducing its coupling with receptors. In striatal membranes, treatment with 40 μg toxin per mg membrane protein labeled 60% of the G_i present and 70% of another G protein, G_o; this treatment reduced binding of the opioid agonist [³H]d-Ala²-d-Leu⁵-enkephalin ([³H]DADLE) 20-50%, with the decrease largerly reflecting a decreased affinity. In cortex, toxin treatment reduced [³H]DADLE binding by 35-70%, corresponding to ADP-ribosylation of 50% of G_i and 40% of G_o. In midbrain, [³H]DADLE binding was unaffected by toxin treatment that ADP-ribosylated 86% of the G_i and 72% of the G_o. These results provide further evidence that opioid receptors are associated with GTP-binding proteins in striatum and cortex, where they have also been shown to inhibit adenylate cyclase. Despite the presence of G_i and G_o in midbrain, however, there appears to be no coupling between them and opioid receptors.