Stimulation of guanosine-5'-o-(3-[35S]thio)triphosphate binding in digitonin-permeabilized C6 rat glioma cells: evidence for an organized association of mu-opioid receptors and G protein.

The guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding assay for the determination of relative opioid efficacy has been adapted to measure G protein activation in digitonin-permeabilized C6 rat glioma cells expressing a cloned mu-opioid receptor. The mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) caused a 3-fold increase in [35S]GTPgammaS binding over basal in a naloxone-sensitive manner. Relative mu-agonist efficacy was DAMGO > fentanyl > or = morphine > buprenorphine. Nalbuphine showed no efficacy. G protein activation by receptors has been predicted to occur by random encounter. In this model a reduction in the number of receptors will decrease the rate of G protein activation but not the maximum number of G proteins activated. To test this model C6 mu cells were treated with the irreversible mu-antagonist beta-funaltrexamine (10 nM) prior to permeabilization. This reduced the number of mu-opioid receptors determined with [3H]diprenorphine to 23 +/- 3% of control with no change in affinity. A commensurate reduction (to 29 +/- 10% of control) in the level of [35S]GTPgammaS binding stimulated by DAMGO was observed, but the t(1/2) for [35S]GTPgammaS binding remained unchanged. Thus, random encounters of receptor and G protein failed to occur in this permeabilized cell preparation. A model that assumes an organized association of G proteins with receptors better describes the activation of G proteins by opioid mu-receptors.