Chronoamperometry To Determine Differential Reductions in Uptake in Brain Synaptosomes from Serotonin Transporter Knockout Mice

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Abstract
The serotonin transporter (SERT) is a neuronal plasma membrane protein whose primary function is to take up the neurotransmitter serotonin from the extracellular space, thereby controlling the spatial and temporal aspects of serotonergic signaling in the brain. In humans, a commonly expressed genetic variant of the serotonin transporter gene results in 40% reductions in SERT expression that have been linked to increases in anxiety-related personality traits and susceptibility to stress-associated depression. Mice have been engineered to express similar reductions in SERT expression to investigate transporter-mediated control of serotonin neurotransmission and behavior. We employed carbon fiber microelectrode voltammetry (chronoamperometry) to examine serotonin clearance rates in brain liposomes (synaptosomes) prepared from mice with 50% (SERT+/-) or complete (SERT-/-) loss of SERT expression. Initial characterization of uptake showed that transport of serotonin was enhanced in the presence of oxygen and abolished when synaptosomes were stirred. Additionally, uptake was prevented by inclusion of the serotonin-selective reuptake inhibiting drug paroxetine in the incubation medium. Most notably, unlike prior studies using established radiochemical methods in synaptosomes, we determined 60% reductions in serotonin uptake rates in SERT+/- mice in two different brain regionsstriatum and frontal cortex. Serotonin uptake was not detected in either brain region in SERT-/- mice. Thus, electroanalytical methods offer distinct advantages stemming from excellent temporal resolution for determining transporter kinetics. Moreover, these appear necessary for delineating moderate but biologically important changes in neurotransmitter transporter function.