Voltage and ionic regulation of human serotonin transporter in Xenopus oocytes

SUMMARY 1 The serotoninergic system is known to be involved in the control of multiple behavioural and physiological functions. The serotonin (5-hydroxtryptamine; 5-HT) transporter (SERT), which controls the synaptic 5-HT concentration through re-uptake of this neurotransmitter into presynaptic terminals, has been a primary therapeutic target for various psychiatric and peripheral disorders. The aim of the present study was to identify the regulatory mechanism(s) of the human SERT (hSERT) in heterologously expressed oocytes. 2 The hSERT cRNA was transcribed in vitro and injected into Xenopus oocytes. The 5-HT-induced transporter currents were measured by voltage clamp. The effects of extracellular sodium or chloride were studied by replacement perfusion with tetramethylammonium-chloride (96 mmol/L) or sodium acetate (96 mmol/L). In addition, to alter the internal calcium concentration, CaCl2 (50 µmol/L) and inositol triphosphate (IP3; 50 µmol/L), with or without EGTA (2.5 mmol/L), were injected into oocytes. The specificity of 5-HT-sensitive currents was determined by the use of the SERT antagonist desipramine and niflumic acid to block background chloride currents. 3 The hSERT-expressing oocytes displayed voltage-dependent, 5-HT-induced currents that increased at negative potentials. Replacing extracellular sodium or chloride significantly decreased the hSERT currents by 89 and 45%, respectively (P < 0.05, n = 7 each). Injection of IP3 or CaCl2 increased the hSERT currents by approximately 65% (P < 0.05; n = 10 each) and the effect of IP3 was abolished by preinjection of EGTA. 4 These results demonstrate that hSERT activity is not only voltage dependent, but is also affected by intracellular calcium and extracellular sodium and chloride.