Heterogeneities in Axonal Structure and Transporter Distribution Lower Dopamine Reuptake Efficiency

Cihan Kaya, Mary H. Cheng, Ethan R. Block, Tom M. Bartol, Terrence J. Sejnowski, Alexander Sorkin, James R. Faeder, Ivet Bahar
2018 eNeuro  
Efficient clearance of dopamine (DA) from the synapse is key to regulating dopaminergic signaling. This role is fulfilled by DA transporters (DATs). Recent advances in the structural characterization of DAT from Drosophila (dDAT) and in high-resolution imaging of DA neurons and the distribution of DATs in living cells now permit us to gain a mechanistic understanding of DA reuptake events in silico. Using electron microscopy images and immunofluorescence of transgenic knock-in mouse brains that
more » ... express hemagglutinin-tagged DAT in DA neurons, we reconstructed a realistic environment for MCell simulations of DA reuptake, wherein the identity, population and kinetics of homology-modeled human DAT (hDAT) substates were derived from molecular simulations. The complex morphology of axon terminals near active zones was observed to give rise to large variations in DA reuptake efficiency, and thereby in extracellular DA density. Comparison of the effect of different firing patterns showed that phasic firing would increase the probability of reaching local DA levels sufficiently high to activate low-affinity DA receptors, mainly owing to high DA levels transiently attained during the burst phase. The experimentally observed nonuniform surface distribution of DATs emerged as a major modulator of DA signaling: reuptake was slower, and the peaks/width of transient DA levels were sharper/wider under nonuniform distribution of DATs, compared with uniform. Overall, the study highlights the importance of accurate descriptions of extrasynaptic morphology, DAT distribution, and conformational kinetics for quantitative evaluation of dopaminergic transmission and for providing deeper understanding of the mechanisms that regulate DA transmission. Significance Statement Dopamine (DA) modulates motor control, cognition, and drug addiction. Understanding the mechanism of dopamine transmission is essential to designing therapies for neurologic disorders. We developed a multiscale model using advances in imaging and high-performance computing technologies, which permitted us to perform spatially realistic simulations of dopamine reuptake. Simulations show large temporal and spatial variations in the local density of dopamine depending on the morphology of the synaptic/ extrasynaptic region near dopamine release site and on firing pattern. Dopamine clearance is less efficient under heterogeneous distribution of dopamine reuptake transporters, compared with a uniform distribution with the same average surface density. Dopamine reuptake transporters membrane distribution, accessibility of dopamine reuptake transporters outward-facing conformation, and large fluctuations in dopamine levels emerge as key features that modulate dopaminergic transmission.
doi:10.1523/eneuro.0298-17.2017 pmid:29430519 pmcid:PMC5804147 fatcat:fjxjeocpzrhwfplkzo3ic5zufy