Abstract
Receptor binding studies of 5,14-O-dimethyloxymorphone (14-methoxymetopon) in brain membranes have established its high affinity for μ-binding sites, but its analgesic potency far exceeds the modest increase in binding affinity relative to other opioids. The current study has established the selectivity of [3H]14-methoxymetopon for μ sites in calf striatal membranes and for a number of full-length splice variants of the cloned murine μ-opioid receptor 1 (mMOR-1) in transfected cell lines. The binding affinity of [3H]14-methoxymetopon for the variants expressed in Chinese hamster ovary cells was quite high, with KD values around 0.2 nM for all of the variants with the exception of mMOR-1F (KD of 1.2 nM). The affinity for most of the expressed variants was greater than that seen in the brain membranes (KD of 0.99 nM). Functionally, in guanosine 5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assays with the MOR-1 variants, 14-methoxymetopon and the μ-opioid peptide [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) showed similar efficacies, as determined by maximal stimulation, but 14-methoxymetopon was up to 65-fold more potent than DAMGO. The greatest difference was seen with mMOR-1E and the least with mMOR-1C, which displayed only a 10-fold difference. These potency differences in the stimulation of [35S]GTPγS binding far exceeded the differences in binding affinity. The differences between 14-methoxymetopon and DAMGO remained after normalizing the potency shifts based upon receptor binding affinities and varied from 1.2-fold with mMOR-1C to 21-fold for mMOR-1 and 42-fold with mMOR-1F. Thus, 14-methoxymetopon is a potent agonist against all of the mMOR-1 splice variants, but its potency ranged widely despite similar binding affinities for most of the variants and may give insight into its unusual pharmacological profile.
Footnotes
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This work was supported, in part, by Grants DA02615 and DA07242 and Senior Scientist Award DA00220 (to G.W.P.) from the National Institute on Drug Abuse; core Grant CA08748 (to Memorial Sloan-Kettering Cancer Center) from the National Cancer Institute; and OTKA TS 049817 (to G.T.) from the Hungarian National Research Foundation.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.106.105395.
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ABBREVIATIONS: M6G, morphine-6β-glucuronide; MOR, μ-opioid receptor; DAMGO, [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin; GTPγS, guanosine 5′-O-(3-thio)triphosphate; CHO, Chinese hamster ovary; Gpp(NH)p, guanosine 5′-(β,γ-imido)triphosphate; U50,488H, (1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-ly]-benzeneacetamide.
- Received March 28, 2006.
- Accepted June 7, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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