Measurement of transient currents from neurotransmitter transporters expressed in Xenopus oocytes

Electrophysiological measurements add new dimensions to the study of neurotransmitter transporters. (1) One can perform measurements with high temporal resolution (however, uptake of radioactive substrate is limited in that it cannot resolve events that occur within 1 sec, which is greater than the time of a single transport cycle). (2) Electrophysiology provides information about partial steps in transport cycles, including the fact that ion binding and dissociation at transporters can generate currents, which provides new insights about ion-transporter interaction. (3) Electrophysiology provides information about single transporter molecules, from patch-clamp recordings of single-channel activity of neurotransmitter transporters. At present, little is known about the molecular mechanisms that underlie transport. Electrophysiological measurements of ion binding and permeation contribute to the analysis of mutations that affect transport. Electrophysiology may help to identify amino acids and domains in neurotransmitter transporters that participate in specific ways in the transport process, such as ion neurotransmitter binding, permeation pathways, voltage sensors, and gates. In combination with spectroscopic measurements, it may also be possible to identify the actual conformational changes of the proteins that enable substrate translocation.