Substrates and temperature differentiate ion flux from serotonin flux in aserotonin transporter

Neurotransmitter transporters couple the transport of transmitter against i ts concentration gradient to the electrochemical potential of associated io ns which are also transported. Recent studies of some neurotransmitter tran sporters show them to have properties of both traditional carriers and subs trate-dependent ion channels, in that ion fluxes are in excess of that pred icted from stoichiometric substrate fluxes. Whether these properties are co mparable for all transporters, the extent to which these permeation states are independent, and whether the relationship between these two states can be regulated are not well understood. To address these questions, we expres sed the Drosophila serotonin (5HT) transporter (dSERT) in Xenopus oocytes a nd measured both substrate-elicited ion flux and SPIT flux at various tempe ratures and substrate concentrations. We find that the ion flux and SPIT fl ux components of the transport process have a significant temperature depen dence suggesting that ion flux and transmitter flux arise from a similar th ermodynamically-coupled process involving large conformational changes (e.g ., gating). These data are in contrast to those shown for glutamate transpo rters, suggesting a different permeation process for 5HT transporters. The relationship between ion flux and 5HT flux is differentially regulated by c hloride and 5HT, suggesting that these permeation states are distinct. The difference in half-maximal 5HT concentration necessary to mediate ion flux and SPIT flux occurs at submicromolar SPIT concentrations suggesting that t he relative participation of dSERT in ion flux and SPIT flux will be determ ined by the synaptic SPIT concentration. (C) 2001 Elsevier Science Ltd. All rights reserved.