REGULATION OF THE HUMAN SEROTONIN TRANSPORTER - CHOLERA TOXIN-INDUCEDSTIMULATION OF SEROTONIN UPTAKE IN HUMAN PLACENTAL CHORIOCARCINOMA CELLS IS ACCOMPANIED BY INCREASED SEROTONIN TRANSPORTER MESSENGER-RNA LEVELS AND SEROTONIN TRANSPORTER-SPECIFIC LIGAND-BINDING

Treatment of confluent cultures of JAR human placental choriocarcinoma cells with cholera toxin or forskolin for 16 h markedly stimulated (2 .4-fold) serotonin transport activity in these cells. Cycloheximide, a n inhibitor of protein synthesis or actinomycin D, an inhibitor of mRN A synthesis effectively blocked this stimulation. Northern blot analys is revealed that treatment with cholera toxin resulted in severalfold increase in the concentrations of the three mRNA species (6.8, 4.9 and 3.0 kilobases in size) which hybridized to the human placental seroto nin transporter cDNA. Under similar conditions, the concentrations of the mRNA species which hybridized to the human placental taurine trans porter cDNA or to the human beta-actin cDNA were not affected. Analysi s of paroxetine-sensitive binding of the cocaine analog ta-carbomethox y-3beta-(4-[I-125]iodophenyl)tropane to the membranes prepared from co ntrol and cholera toxin-treated cells indicated that the maximal bindi ng capacity was increased 2.5-fold by cholera toxin, with no significa nt change in the binding affinity. Thus, stimulation of serotonin tran sporter activity in the placental choriocarcinoma cells following chol era toxin treatment is likely a result of an increase in cell surface density of the serotonin transporter protein as a consequence of incre ased steady state serotonin transporter mRNA levels.