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C.L. Zucker, J. Nilson; Hemicholinium-3-sensitive High-affinity Choline Uptake Transporter Defines Acetylcholine Release Sites on Starburst Amacrine Cell Dendrites . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4142.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In recent years, there has been keen interest in deciphering the mechanism underlying directionally selective (DS) properties in ganglion cells. Central to the generation of such properties are two mirror-symmetric populations of starburst amacrine cells (SbAC) whose dendrites ramify in two distinct sublaminae, overlapping those of the ON-OFF DS ganglion cells. Using both acetylcholine and GABA as neurotransmitters, these SbAC receive significant glutamatergic input from bipolar cells and provide synaptic output to DS ganglion cells. In addition, other ganglion and amacrine cell types are also cholinoreceptive, suggesting that acetylcholine likely plays multiple roles in retinal processing. Most models generated to explain DS properties invoke some form of asymmetric interaction between excitation and inhibition on the ganglion cells dendrites. Recent studies now suggest that asymmetries necessary to encode "preferred" and "null" directions may reside in individual limbs of SbAC dendrites. Methods: In order to further define the microcircuitry used by these cells to carry out their neural computations, we have been utilizing intracellular filling of SbAC combined with confocal visualization of immunohistochemically localized hemicholinium-3 sensitive high-affinity choline uptake transporter (CHT1). High-affinity choline uptake into SbAC presynaptic terminals is the rate-limiting step in acetylcholine synthesis. Additionally, ultrastructural analysis of CHT1 labeling has been performed. Results: We have found that CHT1 labeling precisely corresponds to the two laminae labeled by choline acetyltransferase antibodies. Cell body labeling is highly limited to the perinuclear region. Lucifer yellow filled SbAC exhibit discrete CHT1 labeling at dendritic spines and varicosities. The majority of these labeled sites are found on the distal two thirds of the SbAC dendritic tree. Electron microscopy shows enhanced CHT1 labeling in the vicinity of synaptic profiles. When compared to the localization of choline acetyltransferase, such restricted localization allows for greater definition of synaptic relationships between SbAC and their synaptic partners. Conclusions: It is likely that such discrete sites of high-affinity choline uptake also represent the locations of acetylcholine release. Defining the loci of acetylcholine release as well as sites of synaptic interactions near such release sites, provides for a greater understanding of the structural attributes that lead to DS properties in retinal ganglion cells.
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