RT Journal Article SR Electronic T1 Identification and Characterization of Three New Alternatively Spliced μ-Opioid Receptor Isoforms JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 396 OP 403 DO 10.1124/mol.56.2.396 VO 56 IS 2 A1 Ying-Xian Pan A1 Jin Xu A1 Elizabeth Bolan A1 Catherine Abbadie A1 Albert Chang A1 Amy Zuckerman A1 Grace Rossi A1 Gavril W. Pasternak YR 1999 UL http://molpharm.aspetjournals.org/content/56/2/396.abstract AB We have identified four new μ-opiod receptor (MOR)-1 exons, indicating that the gene now contains at least nine exons spanning more than 200 kilobases. Replacement of exon 4 by combinations of the new exons yields three new receptors. When expressed in Chinese hamster ovary cells, all three variants displayed high affinity for μ-opioid ligands, but κ and δ drugs were inactive. However, there were subtle, but significant, differences in the binding profiles of the three variants among themselves and from MOR-1. Immunohistochemically, the major variant, MOR-1C, displayed a regional distribution quite distinct from that of MOR-1. Region-specific processing also was seen at the mRNA level. Antisense mapping revealed that the four new exons were all involved in morphine analgesia. Together with two other variants generated from alternative splicing of exon 4, there are now six distinct MOR-1 receptors.