Differential G-protein activation by alkaloid and peptide opioid agonists in the human neuroblastoma cell line SK-N-BE

Differences in the specificity of coupling of delta-opioid receptor with G-protein have been reported in the literature. We have observed a differential desensitization of delta-opioid receptors, endogenously expressed in the neuroblastoma cell line SK-N-BE, induced by peptide and alkaloid agonists. By combining photoaffinity labelling of receptor-activated G-proteins with [alpha-P-32]-azidoanilide-GTP and an anti-sense oligodeoxynucleotide strategy, we examined whether the chemical nature of opioid agonists, alkaloid or peptide, has a critical role in determining a G(i)alpha/G(o)alpha-protein-selective activation by the human delta-opioid receptors. Etorphine, a non-selective alkaloid agonist, was shown to stimulate the incorporation of [alpha-P-32]azidoanilide-GTP into G(i)alpha 1, G(i)alpha 2, G(i)alpha 3 and pertussis-toxin-insensitive G alpha subunits. In contrast, [D-Pen(2),D-Pen(5)]enkephalin (DPDPE; Pen is penicillamine) and Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 (deltorphin I), selective peptide agonists, mainly activated G(i)alpha 2 and G(o)alpha 2 subunits. The 'knock-down' of G(o)alpha 2 subunits by anti-sense oligodeoxynucleotides selectively decreased the inhibition of adenylate cyclase induced by DPDPE and deltorphin I, whereas anti-sense oligodeoxynucleotides directed against G(i)alpha 2 subunits only decreased the potency of etorphine in inhibiting cAMP accumulation. These results suggest that the nature of the agonist, peptide or alkaloid is critical in determining the interaction between human delta-opioid receptors and G alpha subunits.