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 Gi, reducing its coupling with receptors. In striatal membranes, treatment with 40 micrograms toxin per mg membrane protein labeled 60% of the Gi present and 70% of another G protein, Go; this treatments reduced binding of the opioid agonist [3H]D-Ala2-D-Leu5-enkephalin ([3H]DADLE) 20-50%, with the decrease largely reflecting a decreased affinity. In cortex, toxin treatment reduced [3H]DADLE binding by 35-70%, corresponding to ADP-ribosylation of 50% of Gi and 40% of Go. In midbrain, [3H]DADLE binding was unaffected by toxin treatment that ADP-ribosylated 86% of the Gi and 72% of the Go. 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 Gi and Go in midbrain, however, there appears to be no coupling between them and opioid receptors.