S.A. Schnell1, M.W. Wessendorf1, M. Ramsden2, A. Kalyuzhny2
Many immunofluorescent studies of the µ Opioid Receptor (OPRM1), also referred to as MOR, use antisera directed against the carboxy (C-) terminal region to localize OPRM1 in tissue sections. These antisera share common epitopes, are usually not species-selective, and no labeling is observed with them in mice that do not express functional OPRM1. We have developed an antisera directed against the rat OPRM1 amino (N-) terminal sequence that appears to recognize only rat and not mouse OPRM1.
We used two-color immunofluorescence to determine the spatial relationships in spinal cord between OPRM1 labeling obtained with the N- and C-terminal epitopes in rats and mice. Animals were perfused with Lana’s fixative and then sections were stained using a Rabbit Anti-Rat µ Opioid Receptor/OPRM1 Monoclonal Antibody (R&D Systems, Catalog # MAB8629) directed at the receptor's N-terminus, and a guinea pig anti-rat OPRM1 antiserum containing antibodies directed at the C-terminus of the receptor. These were followed by cyanine Cy™ 2-conjugated donkey anti-rabbit and cyanine Cy™ 3-conjugated donkey anti-guinea pig secondary antibodies.
We observed that virtually all cell bodies, fibers, and puncta were double-labeled in rat spinal cord. Additionally, we observed that, in rats, the two antibodies appeared to label identical subcellular structures. In contrast, only the C-terminal antibody stained cell bodies, fibers, and puncta in tissue sections of mouse spinal cord. With both antibodies, staining was abolished in tissue sections where the antisera were pre-incubated with their respective immunizing peptides. The basis for this species-specificity is unclear since the amino acid sequences of the N-terminal region of rat and mouse OPRM1 are nearly identical.
These studies were supported by R&D Systems, Inc. and the Department of Neuroscience at the University of Minnesota.