PT  - JOURNAL ARTICLE
AU  - Mary J. Clark
AU  - Cheryse A. Furman
AU  - Timra D. Gilson
AU  - John R. Traynor
TI  - Comparison of the Relative Efficacy and Potency of μ-Opioid Agonists to Activate Gα<sub>i/o</sub> Proteins Containing a Pertussis Toxin-Insensitive Mutation
AID  - 10.1124/jpet.105.096818
DP  - 2006 May 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 858--864
VI  - 317
IP  - 2
4099  - http://jpet.aspetjournals.org/content/317/2/858.short
4100  - http://jpet.aspetjournals.org/content/317/2/858.full
SO  - J Pharmacol Exp Ther2006 May 01; 317
AB  - Pertussis toxin (PTX)-insensitive mutants of Gαi/o proteins expressed in C6μ cells were used to examine the hypothesis that there are agonist-specific conformational states of the μ-opioid receptor with coupling preferences to different Gαi/o subtypes, as measured by the degree of stimulation of [35S]guanosine 5′-O-(3-thio)triphosphate (GTPγS) binding. Binding of [35S]GTPγS to endogenous Gαi/o proteins stimulated by the full μ-opioid agonist [d-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO) was completely blocked by overnight treatment with 100 ng/ml PTX. Treatment for 4 h with lower concentrations led to a PTX-dependent reduction in the maximal effect of DAMGO but no alteration in the potency of DAMGO or morphine nor in the relative maximal effect (relative efficacy) of the partial agonists morphine and buprenorphine compared with the full agonist DAMGO. Using PTX-insensitive Gα mutants in which the PTX-sensitive cysteine was replaced with isoleucine, the potency for a series of μ-opioid agonists was highest in cells expressing Gαi3 and Gαo and lowest with Gαi1 and Gαi2, with no significant change in the order of potency, namely, etorphine >> endomorphin-1 = DAMGO = endomorphin-2 = fentanyl = morphine >> meperidine. The order of agonist relative efficacy, etorphine = DAMGO = endomorphin-1 = endomorphin-2 = fentanyl ≥ morphine ≥ meperidine > buprenorphine ≥ nalbuphine, was also the same across all of the PTX-insensitive Gαi/o subtypes. Highest relative efficacy to stimulate [35S]GTPγS binding was seen with Gαi3. Consequently, reported observations of agonist-directed trafficking at μ-opioid receptors most likely involve non-PTX-sensitive Gα protein mechanisms. The American Society for Pharmacology and Experimental Therapeutics