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S.C. Mangel, J.E. Nilson, K.E. Gavrikov, A.V. Dmitriev, C.L. Zucker; Dendritic Compartmentalization of Chloride Cotransporters on Dendrites of Starburst Amacrine Cells Underlies Direction Selectivity . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2338.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Purpose: Directionally–selective (DS) light responses occur first in the retina in the dendrites of starburst amacrine cells (SACs). However, the mechanism that underlies the DS responses of SAC dendrites is unclear. The Na–K–Cl (NKCC) and K–Cl (KCC) cotransporters can mediate the depolarizing and hyperpolarizing effects of GABA, respectively. Recent evidence indicates that blockade of the Na–K–Cl and K–Cl cotransporters by bumetanide (BMN) and furosemide (FUR), respectively, eliminates the DS responses of SAC dendrites (Gavrikov et al., 2003). We therefore studied whether the differential distribution of NKCC and KCC on SAC dendrites, which would result in a GABA–evoked depolarization and hyperpolarization in different dendritic compartments, underlies their DS responses. Methods: Whole–cell patch clamp recordings of DAPI–stained displaced rabbit SACs were obtained and the effects of BMN, FUR, and GABA studied. The staining patterns of NKCC and KCC on SAC dendrites were determined on ChAT–labeled and Alexa–filled cells. Results: When BMN (10 µM) and FUR (25 µM) were introduced into SACs via the patch pipettes, the DS light responses of SAC dendrites were eliminated. Moreover, FUR consistently depolarized (ave = 4.2 +/– 1.5 (SEM) mV, n = 28) and BMN consistently hyperpolarized (ave = –17.3 +/– 2.6 mV, n =13) the cells. GABA application with puff pipettes onto either the SAC proximal dendrite or onto the distal dendrite approx 100 µm from the cell body after synaptic transmission was blocked with cobalt indicated that the GABA reversal potential of the starburst proximal and distal dendrites were – 50.6 +/– 2.4 mV and – 60.4 +/– 2.9 mV (p < 0.01, n = 7), respectively. ChAT labeled SACs showed specific NKCC2 and KCC2 labeling. Quantification of the relative distribution of the cotransporters along the dendrites of Alexa–filled SACs showed a highly significant difference (p < 0.0001) between the proximal and distal compartments for both NKCC2 and KCC2 with NKCC2 predominating proximally and KCC2 predominating distally. Conclusions: These results demonstrate that SAC dendrites express both NKCC2 and KCC2 and that the cotransporters work together, but in opposite fashion, to generate the directional responses of SAC dendrites. The results also show that the differential distribution of NKCC2 and KCC2 along the length of starburst dendrites underlies their DS light responses.
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