Immunohistochemical localization of the mu-opioid receptor in the bovine uterus

Endogenous opioid peptides play an important role in the regulation of the reproductive system. These peptides act through three main subtypes of opioid receptors: μ, δ, κ. The μ-opioid receptor (MOR) is the primary site of action of several endogenous opioid peptides. The expression of MOR protein has been found in the bovine uterus, but its cellular location is not known. The aim of this study was to localize the MOR in the bovine uterus by means of immunohistochemistry. Tissue fragments were fixed overnight in 4% (w/v) phosphate-buffered paraformaldehyde at 4 °C and routinely embedded in paraffin wax. Sections 4 mm thick were cut and, after de-waxing, they were incubated in a 1:1500 dilution of primary rabbit polyclonal antibody against the third extracellular loop of the mouse MOR (Chemicon, Temecula, CA, USA). Immunohistochemical visualisation was obtained using the Vecta-lab “Elite” (ABC) kit (Vector, Burlingame, CA) and by incubating with a DAB solution. MOR was revealed in some scattered cells along the epithelium lining the endomentrium which showed immunoreactivity in most stromal cells. Immunoreactivity was also detected in muscle cells of myometrium and blood vessels. Since the expression of MOR was observed in both the endometrium and the myometrium, it is possible to infer that in bovine uterus, as in other mammals (Zhu and Pintar 1998, Biol. Reprod. 195: 451-5), the endogenous opioid system could be implicated in the embryo-maternal interaction, in the uterine contractility and in the analgesia