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Cocaine-sensitive monoamine transporters in rat placenta labeled with (iodine-125)RTI-55 and tritiated nisoxetine

Lauren Patricia Shearman, University of Massachusetts Amherst


Some of the adverse effects of prenatal cocaine exposure have been associated with uteroplacental vasculature, and (3H) cocaine binding sites were previously reported in human placental villus tissue (Ahmed et al., 1990). Specific uptake systems for serotonin (5-HT) and norepinephrine (NE) are present in human placental syncytiotrophoblastic cells and may be substrates for cocaine action (Cool et al.. 1990; Ramamoorthy et al., 1993b). To investigate whether monoamine transporters are present in rat placenta, I studied the characteristics of placental cocaine recognition sites, and the influence of maternal cocaine treatment on placental and fetal brain monoamine transporter binding. First, saturation analyses were performed with (125I) RTI-55, a potent cocaine congener, on membrane fractions from gestational day (GD) 20 rat placenta. Results yielded curvilinear Scatchard plots that were resolved by non-linear curve-fitting into high- and low-affinity components. Mean Kd values were 0.29 nM and 7.9 nM for high- and low-affinity binding sites respectively, and resembled those found in adult rat brain. Drug displacement studies with monoamine uptake inhibitors indicated that the majority of high-affinity (125I) RTI-55 binding was to the 5-HT transporter. The selective ligands (3H) GBR 12935 and (3H) nisoxetine were used to ascertain whether the rat placenta also possessed dopamine (DA) and/or NE transporters. No saturable binding was detected with (3H) GBR 12935. In contrast, the NE transporter-selective ligand (3H) nisoxetine labeled a single population of binding sites with a Kd of 1.0 nM and a Bmax of 1.2 pmol/mg protein. Drug competition studies confirmed that (3H) nisoxetine labeled NE transporters in rat placenta. Additionally, monoamine transporter binding was localized in GD 20 placenta using in vitro film and dry emulsion autoradiography. (3H) Nisoxetine-labeled NE transporters were visible throughout the placenta, with high densities in giant trophoblastic cells and moderate labeling in the labyrinth. Cocaine-like binding sites labeled with (125I) RTI-55 were mainly distributed in the labyrinth and decidua. To verify the presence of 5-HT transporters in rat placenta, 5-HT immunocytochemistry was carried out. The distribution of 5-HT-immunoreactive cells paralleled sites of (125I) RTI-55 binding in the rat placenta. In the final experiment, pregnant dams received cocaine continuously via s.c. Silastic implants from GD 17 to GD 20, and changes in fetal brain and placental (125I) RTI-55 and (3H) nisoxetine binding were assessed. Cocaine treatment did not alter (125I) RTI-55 binding in the placenta. No effect was found in fetal brain, whereas cocaine produced a marked upregulation of NE transporter density in the placenta at GD 20. These studies demonstrate that rat placenta, a noninnervated tissue, possesses cocaine-like binding sites and 5-HT and NE transporters. Placenta is thus a potential target for mediating some effects of cocaine on fetal development.

Subject Area


Recommended Citation

Shearman, Lauren Patricia, "Cocaine-sensitive monoamine transporters in rat placenta labeled with (iodine-125)RTI-55 and tritiated nisoxetine" (1996). Doctoral Dissertations Available from Proquest. AAI9619437.