Mu and Delta opioid receptors activate the same G proteins in human neuroblastoma SH‐SY5Y cells

There is evidence for interactions between mu and delta opioid systems both in vitro and in vivo . This work examines the hypothesis that interaction between these two receptors can occur intracellularly at the level of G protein in human neuroblastoma SH‐SY5Y cells. The [ 35 S]GTPγS binding assay was used to measure G protein activation following agonist occupation of opioid receptors. The agonists DAMGO (EC 50 , 45 n M ) and SNC80 (EC 50 , 32 n M ) were found to be completely selective for stimulation of [ 35 S]‐GTPγS binding through mu and delta opioid receptors respectively. Maximal stimulation of [ 35 S]‐GTPγS binding produced by SNC80 was 57% of that seen with DAMGO. When combined with a maximally effective concentration of DAMGO, SNC80 caused no additional [ 35 S]‐GTPγS binding. This effect was also seen when measured at the level of adenylyl cyclase. Receptor activation increased the dissociation of pre‐bound [ 35 S]‐GTPγS. In addition, the delta agonist SNC80 promoted the dissociation of [ 35 S]‐GTPγS from G proteins initially labelled using the mu agonist DAMGO. Conversely, DAMGO promoted the dissociation of [ 35 S]‐GTPγS from G proteins initially labelled using SNC80. Tolerance to DAMGO and SNC80 in membranes from cells exposed to agonist for 18 h was homologous and there was no evidence for alteration in G protein activity. The findings support the hypothesis that mu‐ and delta‐opioid receptors share a common G protein pool, possibly through a close organization of the two receptors and G protein at the plasma membrane.British Journal of Pharmacology (2002) 135 , 217–225; doi: 10.1038/sj.bjp.0704430