Compartmental distribution of two cation chloride cotransporter types along starburst amacrine cell dendrites underlies the directional properties of these dendrites
Nilson, James E.
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A fundamental aspect of vision is the ability to detect motion and to define its direction. In the retina, directionally selective ganglion cells respond to stimulus motion in a 'preferred' direction but respond little to stimulus motion in the opposite or 'null' direction. However despite nearly forty years of investigation, the precise cellular locus and underlying mechanisms of direction selective encoding have remained largely elusive. Recently, starburst amacrine cells, that are presynaptic to directionally selective ganglion cells, have been shown to provide direction specific inhibitory output to these ganglion cells. Therefore defining the biophysical properties specific to starburst amacrine cell dendrites will provide significant insight into the ability of visual systems to encode the direction of objects moving through an animal's visual field. Using a combination of intracellular filling of starburst amacrine cells and immunohistochemical localization of biophysically relevant molecules, we have examined how individual dendrites compute such motion. In order to define the relative degree and pattern of colocalization of these markers on filled dendrites we developed a new set of image acquisition and data analysis procedures that have allowed us to define the biophysical signature intrinsic to different portions of starburst amacrine cell dendrites. We have found that sodium-potassium-chloride cotransporter (NKCC2) and potassium-chloride cotransporter (KCC2) are expressed and differentially distributed on the proximal and distal dendritic compartments of starburst amacrine cells, respectively. The functional relevance of the anatomical distribution pattern of these cation-chloride-cotransporter types has been confirmed by others using physiological techniques. In summary, our studies provide a fundamental mechanism through which starburst amacrine cells define motion direction and transmit this information to directionally selective ganglions cells. In addition, our illumination of the basic concept of segregation of functional components to different dendritic compartments will likely prove to be an important theme of neuronal function throughout the nervous system.
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