Distribution of delta and mu opioid receptor mRNA in rodent dorsal root ganglia neurons

• Published in: The European journal of neuroscience Vol. 56; no. 3; pp. 4031 - 4044
• Main Authors: Quirion, Béatrice, Beaulieu, Claudie, Côté, Laurie, Parent, Jean‐Luc, Gendron, Louis
• Format: Journal Article
• Language: English
• Published: France Wiley Subscription Services, Inc 01.08.2022
• Subjects: Analgesics; delta opioid receptor; Dorsal root ganglia; Fibers; Hybridization; Immunofluorescence; in situ hybridization; Morphine; mRNA; mu opioid receptor; Narcotics; Neurons; Opioid receptors (type delta); Opioid receptors (type mu); Pain perception; primary afferent fibres; Receptor mechanisms; rodents; Spinal cord; delta opioid receptor; rodents; dorsal root ganglia; mu opioid receptor; in situ hybridization; primary afferent fibres
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/ejn.15733

Primary afferents are responsible for transmitting signals produced by noxious stimuli from the periphery to the spinal cord. Mu and delta opioid receptors (MOP and DOP) have analgesic properties and are highly expressed in dorsal root ganglia (DRG) neurons. In humans, spinal DOP is almost exclusively located on central terminals of DRG neurons, whereas in rodents, it is expressed both on presynaptic terminals and spinal neurons. In this study, we aimed to assess the distribution of MOP and DOP in the DRGs of mice and rats. Using in situ hybridization and immunofluorescence, we visualized MOP and DOP mRNA together with various neuronal markers. In rats and mice, we show that both receptors are expressed, albeit to different extents, in all types of neurons, namely, large and medium myelinated neurons (NF200‐positive), small nonpeptidergic (IB4‐ or P2X3R‐positive) and peptidergic C fibres (Tac1‐positive). Overall, DOP mRNA was found to be mainly expressed in large and medium myelinated neurons, whereas MOP mRNA was mainly found in C fibres. The distribution of MOP and DOP, however, slightly differs between rats and mice, with a higher proportion of small nonpeptidergic C fibres expressing DOP mRNA in mice than in rats. We further found that neither morphine nor inflammation affected the distribution of the receptor mRNA. Because of their location, our results confirm that MOP and DOP have the potential to alleviate similar types of pain and that this effect could slightly differ between species. In this study, we used RNAscope and immunostaining to study the distribution of delta and mu opioid receptor mRNA in rodents. We show that both receptors are expressed, to different extents, in all types of neurons. Because of their location, we conclude that both receptors have the potential to regulate nociception. Created with BioRender.com