EXPRESSION OF THE OPRM1 GENE IN THE MOUSE STRIATUM

The μ opioid receptor in mice is encoded by the Oprm1 gene. Multiple transcript variants of the gene were identified, products of alternative splicing, two possible transcription starts, and several termination signals. Altogether, about 30 distinct variants encoding several isoforms were reported as possible. Here, we investigated the expression of various transcriptional forms of Oprm1 in the striatum of C57BL/6N mice. We analyzed the results from a long-read Oxford nanopore sequencing together with the data generated by Visium spatial transcriptomics. We established that the most abundant transcript contains exons 1 to 4 and an unexpectedly long, 10 kb 3'UTR in the qA1 region of chromosome 10. To confirm the result, we performed RNAscope in situ hybridization with a customized probe targeting the 3'UTR region. The sequencing also revealed the presence of some variants lacking exon 4, albeit their abundance seemed relatively modest. In conclusion, our data suggest that a dominant, previously not reported Oprm1 transcript in the murine striatum represents approximately 90% of total expression. In our future research, we would like to examine the distribution of the transcripts in other brain regions and potential implications of the long 3'UTR.