DETECTION OF OPIOID RECEPTOR MESSENGER-RNA BY RT-PCR REVEALS ALTERNATIVE SPLICING FOR THE DELTA-OPIOID AND KAPPA-OPIOID RECEPTORS

The three mu-, delta- and kappa-opioid receptors have recently been cl oned and characterized at the molecular level. Our analysis of opioid receptor transcripts by RT-PCR revealed two PCR products derived from delta and kappa mRNAs with size higher than expected from the known cD NA sequences. DNA sequencing showed additional nucleotides inserted be tween the known splice sites, indicating the possible existence of alt ernative splicing pathways for delta and kappa receptors. The novel de lta-opioid receptor transcript is expressed in mouse brain and contain s a 243 bp insertion. This additional sequence is located at the splic e junction between the first and second coding exons and is encoded by a single exon located 9 kb upstream exon 2 in the mDOR gene. The othe r alternative transcript occurs in human monocytic and T lymphocytic c ell lines and encodes a novel form of the kappa-opioid receptor. The P CR product presents a 23 bp deletion at the 3' end of exon 2 followed by a 246 bp insertion found between exons 2 and 3. In the hKOR gene, t his insertion is encoded by two DNA segments. One of them is located 0 .4 kb downstream exon 2 while the second is flanking exon 3 on the 5' side. Both novel putative delta and kappa exons present in-frame stop codons that would lead to truncated receptor proteins. A possible func tional or regulatory role of these shorter proteins in opioid function remains to be determined.