An alternatively spliced transcript of the rat nociceptin receptor ORL1 gene encodes a truncated receptor

Opioid receptor-like protein ORL1, the receptor for the neuropeptide nocice ptin (also named orphanin FQ), has two alternatively spliced isoforms in th e rat. This alternative splicing event is generated by retaining of intron 3, 81 bases in length, in the mRNA region encoding the second extracellular loop of ORL1. A full-length rat ORL1 receptor has 367 amino acid residues. However, as revealed by sequencing of rat ORL1 genomic DNA and cDNA, the i nsertion of the unspliced intron 3 brings in an in-frame stop codon and, th erefore, creates a truncated open-reading frame encoding only the N-termina l half of ORL1 (from the N-terminus to an alternate extracellular tail C-te rminal to the fourth transmembrane domain). The two alternatively spliced t ranscripts are differentially expressed in tissues. In transfected mammalia n cells, the full-length ORL1 displays high-affinity and selective binding for nociceptin, and inhibits the production of cyclic AMP. In contrast, the truncated ORL1 binds nociceptin and other opioid peptides very poorly and non-selectively (affinity in micromolar range), and it does not mediate any inhibitory effects on cyclic AMP production. Apparently, this truncated OR L1 does not function as a receptor for nociceptin or other ligands tested. Such alternative splicing to create a truncated ORL1 receptor might be an e ndogenous mechanism to negatively regulate nociceptin/ORL1 functions. (C) 2 000 Elsevier Science B.V. All rights reserved.