Interaction of fluoxetine with the human placental serotonin transporter

Abstract

The interaction of fluoxetine, a non-tricyclic antidepressant, with the human placental serotonin transporter was investigated by studying its influence on [ 3H]paroxetine binding to the transporter and on [ 3H]serotonin uptake via the transporter. These studies were done using brushborder membrane vesicles purified from normal term human placentas. Fluoxetine inhibited binding of paroxetine to the membrane vesicles in a concentration-dependent manner, with a K i value of 3 nM. Kinetic analysis revealed that the inhibition was competitive because the presence of 10 nM fluoxetine increased the k d for paroxetine from 72 to 461 pM, but had no effect on the B max . Fluoxetine also caused a time-dependent dissociation of paroxetine already bound to the transporter. The dissociation followed first-order kinetics. Uptake of serotonin in these membrane vesicles was also inhibited by fluoxetine. The inhibition was concentration dependent with a K i value of 66 nM at pH 7.5 and 80 nM at pH 6.5. The effect of fluoxetine on the uptake kinetics was to increase the apparent dissociation constant ( K l ) for serotonin without influencing the maximal transport capacity ( V max ). The results demonstrate that fluoxetine is a high-affinity ligand and a potent inhibitor of the serotonin transporter found in the human placental brush-border membrane.