Lack of evidence for the μ-opioid receptor splice variant MOR1_C in rats

We previously reported the existence of MOR1_C mRNA and MOR1_C-immunoreactivity (-ir) in rats. However, the sequence that we reported for rat MOR1C appears not to be present in the rat genome. We have therefore reexamined whether MOR1_C mRNA or MOR1_C-ir exist in rats. We used reverse-transcription polymerase chain reaction (RT-PCR) to attempt to amplify MOR1, MOR1_A, MOR1_B, the rat MOR1_C sequence we previously reported, and MOR1_C1 and MOR1_C2 (which have recently been reported to exist in rats). In RNA extracted from rats, we were able to demonstrate PCR products representing MOR1, MOR1_A, and MOR1_B splice variants. All three products were confirmed as related to MOR1 by Southern blot. However, we were unable to detect either the MOR1_C product reported previously by us or the MOR1_C-like products reported to exist in rats by others. In RNA extracted from mice we were able to detect MOR1, MOR1_A, MOR1_B, and MOR1_D-like products. To test the specificity of our MOR1_C antiserum, we examined MOR1_C-ir in control and knockout mice. MOR1_C-ir had a distribution in control mice similar to that previously reported in rats, including coexisting with vGLUT2. However, although MOR1-ir was absent in MOR1 knockout mice, the density and distribution of MOR1_C-ir were unchanged, suggesting that the antiserum crossreacts with another molecule in tissue. We find no evidence for MOR1_C mRNA in rats. Furthermore, we conclude that MOR1_C-ir represents crossreactivity.