Human placenta as a target organ for cocaine action: Interaction of cocaine with the placental serotonin transporter

The interaction of cocaine and its analog 2β-carbomethoxy-3β-(4-iodophenyl) tropane (RTI-55) with the human placental serotonin transporter was investigated. The function of the placental serotonin transporter was assayed using three different approaches: serotonin uptake in purified human placental brush border membrane vesicles, paroxetine binding in placental brush border membrane vesicles, and serotonin uptake in cultured human placental choriocarcinoma cells. The interaction of cocaine and RTI-55 with the transporter was studied by determining the effects of these compounds on the transporter function. Cocaine and RTI-55 were found to be potent inhibitors of the transporter in all three approaches. The inhibition was competitive in nature in all cases. The inhibition constant (K_i) for cocaine was not influenced significantly by the duration of time allowed for the compound to interact with the transporter. In contrast, the inhibition constant for RTI-55 was influenced markedly by this parameter. The longer the time allowed for interaction of RTI-55 with the transporter, the smaller was the K_i value. These results suggest that the association kinetics for the interaction of cocaine and RTI-55 with the placental serotonin transporter are significantly different. When equilibrium interaction was allowed, cocaine inhibited the function of the transporter with a K_i of 0.09 μm. It is concluded that cocaine and its analog RTI-55 are potent inhibitors of the function of the serotonin transporter that is expressed in the normal human placenta and in cultured human placental choriocarcinoma cells. Since the reported values for cocaine concentration in the blood of cocaine users are several-fold higher than the inhibition constant for cocaine, the present study strongly suggests that the function of the placental serotonin transporter may be severely impaired by maternal use of cocaine during pregnancy. These findings may be relevant to fetal and placental complications of cocaine abuse during pregnancy.