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T. Euler, S. Hausselt, P.B. Detwiler, W. Denk; How Do Starburst Cell Dentrites Become Direction-selective? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4129.
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Purpose: Recent studies strongly support a crucial role for starburst cells in retinal direction selectivity (DS). Local calcium signals in their distal dendrites – but not the somatic voltage – are direction-selective; preferring centrifugal over centripetal motion. Somatic membrane potential modulation to circular stimuli is, however, not easy to reconcile with a simple electrotonic model of dendritic DS. To explore the mechanism of DS in starburst cell dendrites we are studying the spatiotemporal characteristics of calcium responses and their pharmacology in more detail. Methods: Starburst cells in whole mount rabbit retina were filled with calcium indicator via patch-electrodes or iontophoretically with sharp microelectrodes. Using two-photon laser scanning microscopy we optically recorded [Ca++] changes evoked by visual stimuli. Pharmacological agents were applied via the bath (Ames) solution. Results: The velocity tuning curve of dendritic DS peaks at around 1 mm/s and shows a smaller but reversed directional preference for motion faster than 2.5 mm/s. This suggests a fixed phase relationship between response components, e.g. excitation and inhibition. As previously shown, specific GABAA receptor antagonists have no effect on dendritic DS in starburst cells. The specific GABAC receptor antagonist TPMPA ((1,2,5,6-Tetrahydropyridin-4-yl)methylphosphinic acid), however, appears to affect dendritic DS by bringing out [Ca++] modulations independent of the direction of the moving stimulus. The inhibition the cells receive from the far surround persisted, however, under TPMPA application. Conclusions: While blockade of GABAC receptors seems to affect direction selective calcium signals it is not yet clear whether their role is essential for the DS mechanism.
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