Morphine modulates the expression of mu‐opioid receptor exon 5‐associated full‐length C‐terminal splice variants by upregulating miR‐378a‐3p

The mu-opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating an array of splice variants that are conserved from rodent to human. Both mouse and human OPRM1 have five exon 5-associated seven transmembrane full-length carboxyl terminal variants, MOR-1B1, MOR-1B2, MOR-1B3, MOR-1B4, and MOR-1B5, all of which are derived from alternative 3' splicing from exon 3 to alternative sites within exon 5. The functional relevance of these exon 5-associated MOR-1Bs has been demonstrated in mu agonist-induced G protein coupling, adenylyl cyclase activity, receptor internalization and desensitization, and post-endocytic sorting, as well as region-specific expression at the mRNA level. In the present study, we mapped a polyadenylation site for both mouse and human MOR-1Bs that defines the 3'-untranslated regions (3'-UTR) of MOR-1Bs and stabilizes mMOR-1Bs mRNAs. We identified a conserved miR378a-3p sequence in the 3'-UTR of both mouse and human MOR-1BS transcripts through which miR-378a-3p can regulate the expression of MOR-1Bs at the mRNA level. Chronic morphine treatment significantly increased the miR-378-3p level in Be(2)C cells and the brainstem of the morphine tolerant mice, contributing to the decreased expression of the mouse and human MOR-1B3 and MOR-1B4. Our study provides new insights into the role of miRNAs and Oprm1 splice variants in morphine tolerance.