Subject(s)
Cation Transport Proteins , Cations/pharmacology , DNA-Binding Proteins , Ion Channel Gating/drug effects , Potassium Channels, Voltage-Gated , Potassium Channels/metabolism , Trans-Activators , Animals , CHO Cells , Cations/metabolism , Cricetinae , Dose-Response Relationship, Drug , Drug Interactions , ERG1 Potassium Channel , Electric Conductivity , Ether-A-Go-Go Potassium Channels , Humans , Membrane Potentials/drug effects , Potassium/metabolism , Potassium/pharmacology , Potassium Channel Blockers , Potassium Channels/agonists , Potassium Channels/genetics , Sodium/metabolism , Sodium/pharmacology , Transcriptional Regulator ERG , TransfectionABSTRACT
Serotonin transporters (SERTs) are targets for drugs such as Prozac that increase serotonin (5HT) levels by blocking 5HT reuptake. Although SERTs saturate in the micromolar range, synaptic 5HT may exceed 1 mM. To examine SERT's response to high 5HT concentrations, we expressed Drosophila SERT (dSERT) in Xenopus oocytes and found that transport continued to increase with concentration up to 0.3 mM 5HT. As 5HT is a monovalent cation, its entry through an ion channel in SERT might explain uptake at high concentrations. We therefore investigated dSERT using traditional ion channel methods, including mole-fraction experiments under voltage clamp. We propose that SERTs may function as 5HT-permeable channels, and that this mechanism may be important for clearance of the neurotransmitter at high concentrations.
Subject(s)
Carrier Proteins/physiology , Drosophila/physiology , Membrane Glycoproteins/physiology , Membrane Transport Proteins , Serotonin/pharmacology , Animals , Carrier Proteins/genetics , Drosophila Proteins , Female , Lithium/pharmacology , Membrane Glycoproteins/genetics , Membrane Potentials/drug effects , Membrane Potentials/physiology , Nerve Tissue Proteins/physiology , Oocytes/physiology , Paroxetine/pharmacology , Patch-Clamp Techniques , Rats , Serotonin/metabolism , Serotonin/physiology , Serotonin Plasma Membrane Transport Proteins , Sodium/pharmacology , Xenopus laevisABSTRACT
Serotonin (5HT) transporters (SERTs) couple to existing ion gradients to transport 5HT into presynaptic terminals. In mammalian SERTs, the transport cycle is reported as electroneutral, with a translocation of zero net charge, and 5HT uptake is independent of membrane voltage. Yet 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 5HT molecules translocated per ion channel event is unknown. To investigate this, we have used heterologous expression of cloned dSERTs to measure 5HT flux and dSERT currents concurrently under voltage clamp, and we have used fluctuation analysis to measure the size of the elementary ionic events in the same cells. RNA-injected Xenopus oocytes accumulate 5HT, and paroxetine or desipramine inhibit this uptake. RNA-injected oocytes also display paroxetine-sensitive 5HT-induced currents and 5HT-independent leak currents. Na replacement decreases the uptake and the induced currents. 5HT-induced current and 5HT uptake both increase at negative potentials, where 5HT carries approximately 5% of the induced 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, approximately 500 5HT molecules are translocated per channel opening, which, at -20 mV, carries approximately 10,000 electronic charges. These data support a model in which 500 SERT cycles occur for each 5HT-induced channel opening or a model in which 500 5HT molecules and 10,000 electronic charges pass through a common pore.