Expression of MOR_1C-Like μ-Opioid Receptor mRNA in Rats

The reverse transcriptase-polymerase chain reaction (RT-PCR) was used to clone a cDNA fragment of a putative G-protein-coupled receptor from rat brain total RNA. Nucleotide sequencing of this cDNA fragment showed it to be homologous to that of the μ-opioid receptor splice variant MOR_1C from mice. We used the cDNA to make an RNA probe for a ribonuclease protection assay (RPA). The results from the RPA showed a protected fragment of the size expected for MOR_1C mRNA, as well as other RNase-protected fragments that may indicate the existence of other MOR1 transcripts. We then used the RNA probe for in situ hybridization (ISH) experiments. We detected strong autoradiographic labeling over much of the rat telencephalon, diencephalon, mesencephalon, cerebellum, spinal cord, and dorsal root ganglia. These findings suggest that MOR_1C, and possibly other MOR1 splice variants, are important components of the system by which the actions of opioids are transduced.