We investigated the ability of the activated μ-opioid receptor (MOR) to differentiate between myristoylated Gαi1 and GαoA type G proteins, and the maximal activity of a range of synthetic and endogenous agonists to activate each G protein.
Membranes from HEK293 cells stably expressing transfected MOR were chaotrope extracted to denature endogenous G-proteins and reconstituted with specific purified G-proteins. The G subunits were generated in bacteria and were demonstrated to be recognised equivalently to bovine brain purified G protein by CB1 cannabinoid receptors. The ability of agonists to catalyse the MOR-dependent GDP/[35S]GTPS exchange was then compared for Gαi1 and GαoA.
Activation of MOR by DAMGO produced a high-affinity saturable interaction for GαoA (Km=20±1 nM) but a low-affinity interaction with Gαi1 (Km=116±12 nM). DAMGO, met-enkephalin and leucine-enkephalin displayed maximal G activation among the agonists evaluated. Endomorphins 1 and 2, methadone and β-endorphin activated both G to more than 75% of the maximal response, whereas fentanyl partially activated both G-proteins.
Buprenorphine and morphine demonstrated a statistically significant difference between the maximal activities between Gαi1 and GαoA. Interestingly, DAMGO, morphine, endomorphins 1 and 2, displayed significant differences in the potencies for the activation of the two G. Differences in maximal activity and potency, for Gαi1versus GαoA, are both indicative of agonist selective activation of G-proteins in response to MOR activation.
These findings may provide a starting point for the design of drugs that demonstrate greater selectivity between these two G-proteins and therefore produce a more limited range of effects.
British Journal of Pharmacology (2006) 147, 671–680. doi:10.1038/sj.bjp.0706661