DROSOPHILA SEROTONIN TRANSPORTERS HAVE VOLTAGE-DEPENDENT UPTAKE COUPLED TO A SEROTONIN-GATED ION-CHANNEL

Serotonin (5HT) transporters (SERTs) couple to existing ion gradients to transport 5HT into presynaptic terminals. In mammalian SERTs, the t ransport cycle is reported as electroneutral, with a translocation of zero net charge, and 5HT uptake is independent of membrane voltage. Ye t mammalian SERTs exhibit 5HT-induced currents, and Drosophila SERTs ( dSERTs) show voltage-dependent uptake. Thus, the relationship between uptake and current remains controversial; furthermore, the number of 5 HT molecules translocated per ion channel event is unknown. To investi gate this, we have used heterologous expression of cloned dSERTs to me asure 5HT flux and dSERT currents concurrently under voltage clamp, an d we have used fluctuation analysis to measure the size of the element ary ionic events in the same cells. RNA-injected Xenopus oocytes accum ulate 5HT, and paroxetine or desipramine inhibit this uptake. RNA-inje cted oocytes also display paroxetine-sensitive 5HT-induced currents an d 5HT-independent leak currents, Na replacement decreases the uptake a nd the induced currents. 5HT-induced current and 5HT uptake both incre ase at negative potentials, where 5HT carries similar to 5% of the ind uced current. Recently, several groups have reported similar phenomena for other transporters, in which transmitter-induced currents exceed the predictions of coupled transport. We now provide evidence that in dSERT, similar to 500 5HT molecules are translocated per channel openi ng, which, at -20 mV, carries similar to 10,000 electronic charges, Th ese data support a model in which 500 SERT cycles occur for each 5HT-i nduced channel opening or a model in which 500 5HT molecules and 10,00 0 electronic charges pass through a common pore.