Objectives: Clinical observations have indicated that opioids such as morphine, methadone and fentanyl may act through distinct m opioid receptor subtypes. Multiple m receptor subtypes could be responsible for varying analgesic activities and side-effects among opioids. The Oprm gene, encoding the murine m opioid receptor, undergoes extensive alternative splicing as a possible contributor to functional diversity of µ opioid receptors. However, the biological relevance of Oprm splice variants has been controversial, due in part to their very low abundance and the fact that multiple splice forms have been reported by one research group only. The present study aimed at identifying previously reported as well as unrecognized Oprm splice variants in mouse brain.
Methods: µ opioid receptor splice variants were detected from commercially available mouse whole brain cDNA using polymerase chain reaction (PCR) and DNA sequencing.
Results: We have identified 11 of the proposed 17 exons of the Oprm gene and verified the majority of exon combinations used to make up 21 differentially spliced Oprm mRNAs. Notably, a novel splice variant, MOR-1T, demonstrates for the first time that exon 4 can be used in combination with further downstream exons to make up the 3?-end of MOR-1 splice variants. The putative protein encoded by MOR-1T is predicted to be identical to that of the most common form, MOR-1, implying that the MOR-1 protein could be generated from at least five differentially spliced mRNAs.
Conclusion: Our results extend the complexity of Oprm gene splicing, and provide supporting evidence for alternative splicing as a likely major contributor to the diversity of µ opioid receptors.