Identification of three new alternatively spliced variants of the rat mu opioid receptor gene: Dissociation of affinity and efficacy

David A. Pasternak, Ling Pan, Jin Xu, Rui Yu, Ming Ming Xu, Gavril W. Pasternak, Ying Xian Pan

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64 Scopus citations


Mu opioid receptors mediate the pharmacological actions of morphine and morphine-like drugs, such as heroin. The mouse and human Oprm genes undergo splicing. In these present studies, we have identified and characterized three new MOR-1 splice variants from the rat Oprm gene. Using an RT-PCR approach, we isolated the new exons 7, 8 and 9 downstream of exon 3. The rat exons 7 and 9 were homologous to the mouse exons 7 and 9 while the rat exon 8 was not. Northern blot analysis with the new exon probes showed distinctive and abundant transcripts of the variants in the rat brain. Full-length cDNA clones containing the new exons, rMOR-1C1, rMOR-1C2 and rMOR-1D were obtained using an RT-PCR approach. Each contained the same exons 1, 2 and 3 as the original rMOR-1, followed by different combinations of the new exons in place of exon 4. In addition, we also isolated another new variant, rMOR-1A, which contains only exons 1, 2 and 3, and is homologous to the human variant MOR-1A previously identified. All the variants were highly mu-selective in binding studies with little difference in affinities for the mu ligands among the variants. However, functional evaluation of assessments of the variants using agonist stimulated [35S]GTPγS binding assays revealed marked differences among the variants, both in terms of potency and efficacy of the drugs. The relative efficacy of a series of mu opioids to each other varied depending upon the variant studied. Efficacy in the [35S]GTPγS assay did not correlate with either receptor binding affinity or with potency. Thus, selectivity of opioid action might be achieved by designing compounds with varying efficacies at different MOR-1 variants.

Original languageEnglish (US)
Pages (from-to)881-890
Number of pages10
JournalJournal of neurochemistry
Issue number4
StatePublished - Nov 2004
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Cellular and Molecular Neuroscience


  • Alternative splicing
  • GTPgammas
  • MOP
  • MOR-1
  • Morphine
  • Opioid receptor


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