Identification of abundant and evolutionarily conserved opioid receptor circular RNAs in the nervous system modulated by morphine

Takeshi Irie, Rebecca Shum, Ioanna Deni, Amanda Hunkele, Valerie Le Rouzic, Jin Xu, Roger Wilson, Gregory W. Fischer, Gavril W. Pasternak, Ying Xian Pan

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Circular RNAs (circRNAs) are a distinct category of single-stranded, covalently closed RNAs formed by backsplicing. The functions of circRNAs are incompletely known and are under active investigation. Here, we report that in addition to traditional linear mRNAs (linRNA), mouse, rat, and human opioid receptor genes generate exonic circRNA isoforms. Using standard molecular biologic methods, Oprm1 circRNAs (circOprm1) were detected in RNAs of rodent and human brains and spinal cords, as well as human neuroblastoma cells, suggesting evolutionary conservation. Sequencing confirmed backsplicing using canonical splice sites. Oprm1 circRNAs were sense-stranded circRNAs resistant to RNase R digestion. The relative abundance of Oprm1 circRNA to linRNA determined by quantitative reverse transcription polymerase chain reaction varied among mouse brain regions, with circRNA isoforms predominating in rostral structures and less abundant in brain stem. Chronic morphine exposure in mice increased brain circOprm1.e2.3 and circOprm1.e2.e3.e4(302) levels by 1.5- to 1.6-fold relative to linRNA. Sequence analysis predicted numerous microRNA binding sites within Oprm1 circRNA sequences, suggesting a potential role in microRNA sequestration through sponging. In addition, we observed that other opioid receptor genes including d, k, and nociceptin receptor genes produced similar circRNAs. In conclusion, all members of the opioid receptor gene family express circRNAs, with Oprm1 circRNA levels exceeding those of linear forms in some regions.

Original languageAmerican English
Pages (from-to)247-258
Number of pages12
JournalMolecular Pharmacology
Issue number2
StatePublished - 2019
Externally publishedYes

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology


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