Norepinephrine transporters in rat placenta labeled with [3H]nisoxetine

Previous research has identified a norepinephrine (NE) transporter in brush-border membranes from human placental syncytiotrophoblastic cells. In the present study, we used the selective ligand [3H]nisoxetine to demonstrate the presence of an NE transporter in rat placental membranes, determine the binding characteristics of the transporter and ascertain its localization by means of in vitro film and dry-emulsion autoradiography. Additional membrane binding studies were performed with [3H]GBR 12935 to determine whether a dopamine transporter also was present in rat placenta. Saturation analyses carried out on washed membrane fractions from whole rat placentas at gestational day 20 showed saturable [3H]nisoxetine binding (mean Kd = 1.00 nM, Bmax = 1.24 pmol/mg of protein) but no saturable binding of [3H]GBR 12935. When various monoamine uptake inhibitors were tested for their potency to inhibit placental [3H]nisoxetine binding, the results supported the conclusion that the radioligand was labeling an NE transporter. Autoradiographic studies showed the presence of [3H]nisoxetine binding in all three cellular zones of the rat placenta: the decidua, junctional zone and labyrinth. Binding was greatest in the junctional zone, particularly in the giant trophoblastic cells. These findings indicate the presence of a high density of NE transporters in the late-gestation rat placenta. Catecholamine uptake probably has a multifunctional role in placental physiology, and blockade of the NE transporter by certain drugs such as cocaine may therefore contribute to the adverse effects of such compounds on pregnancy outcome and offspring development.