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

J Neurochem. 2004 Nov;91(4):881-90. doi: 10.1111/j.1471-4159.2004.02767.x.


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 [(35)S]GTPgammaS 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 [(35)S]GTPgammaS 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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alternative Splicing / genetics*
  • Animals
  • Binding, Competitive / genetics
  • Blotting, Northern
  • Brain / metabolism
  • CHO Cells
  • Cloning, Molecular
  • Cricetinae
  • Exons / genetics
  • Gene Transfer Techniques
  • Guanosine 5'-O-(3-Thiotriphosphate) / pharmacokinetics
  • Narcotics / pharmacokinetics
  • Rats
  • Receptors, Opioid, mu / biosynthesis
  • Receptors, Opioid, mu / genetics*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Sequence Homology
  • Substrate Specificity


  • Narcotics
  • Receptors, Opioid, mu
  • Guanosine 5'-O-(3-Thiotriphosphate)