Our experience demonstrates that it is difficult to identify endogenous MOPR by immunoblotting, in part due to its low abundance in vivo and heavy and heterogeneous glycosylation, which results in a broad and diffuse band and reduced band intensity [see [26] for review]. Using tissues from MOPR(-/-) mice as the negative control is particularly useful for identification of the receptor since antibodies, even after purification, recognize other non-specific protein bands [18,27]. In the literature, the MOPR bands detected by immunological methods following SDS-PAGE can be divided into two categories: broad and diffuse ones with higher relative molecular masses (Mr's) vs. sharp ones with lower Mr's [For examples, see [44] and [45]]. Researchers including our group have taken multiple approaches to demonstrate the MOPR mainly as a diffuse band by covalent labeling with [3H]beta-funaltrexamine ([3H]beta-FNA)] [29,46], receptor phosphorylation [47-49] and immunoblotting with anti-MOPR antibodies [18,27,30]. The rigor and convergence of pharmacological and biochemical data provide confidence in the unequivocal identification of MOPR [see [26] for review]. This single broad band of the mature MOPR has a Mr range between 58 to
