Expression and immunolocalization of the oxytocin receptor in human lactating and non-lactating mammary glands

The milk ejection reflex is mediated by the release of pituitary oxytocin a nd its interaction with specific receptors within the mammary gland. Althou gh up-regulation of the oxytocin receptor during lactation has been shown f or the rat mammary gland by ligand binding assay, investigation of the rece ptor expression in human breast at the molecular level has not yet been car ried out in detail. Here we report the expression and immunolocalization of the oxytocin receptor in the human breast. It appears that the expression level of the receptor-specific mRNA is not significantly elevated during la ctation and the protein remains at a relatively low level. However, this la ck of increase may be only a dilution effect because of the high level of m ilk protein expression. Immunohistochemistry and immunoelectron microscopy using three anti-oxytocin receptor antibodies raised against different epit opes of the receptor indicated the presence of receptor immunoreactivity on ly to a very limited extent in the myoepithelial cells; more specific expre ssion appeared to occur in the ductal/glandular epithelium in both the non- lactating as well as lactating breast, This finding was also confirmed in a New World monkey, the common marmoset (Callithrix jacchus), These results suggest that, at least for human and marmoset, in addition to-or even inste ad of-myoid cells, the ductal/glandular epithelium is also a target for oxy tocin action, not only during lactation but also in the non-lactating breas t. Thus, there may be other physiological effects of oxytocin besides direc t myoid cell contraction in the breast.