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M. J. Roux, S. Deniz, E. Wersinger, Y. Schwab, A. Rendon, J.-A. Sahel, S. Picaud; Volatility and Restoration of GABA in Mouse Horizontal Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1035.
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In the retina, horizontal cells (HCs) mediate lateral inhibition, a process that reinforces luminance contrast through a feedback on photoreceptors and/or a feed-forward on bipolar cell dendrites. While GABA was initially proposed as the feedback/feedforward transmitter, GABA was only very occasionally found in horizontal cells from adult mammalian retina, despite the expression of GAD and VIAAT. In the present study, we investigated if mouse horizontal cells can produce GABA.
Whole retinas or retina slices were fixed at various stages following incubations in classical Ringer solutions (aCSF, Ames) at 25°C or 37°C, complemented or not with GABA or GABA synthesis precursors. Tissue was then processed for immunohistochemistry with GABA and calbindin antibodies.
When animals were fixed by perfusion with cold fixative, HCs appeared GABA-positive whereas they were GABA-negative when the eye was fixed by mere immersion in fixative. To further analyze if GABA can be produced by HCs, the retina was dissected and incubated in Ames medium at 37°C. GABA was absent from HCs following this procedure. When the slices were incubated in the presence of GABA, Müller cells accumulated GABA but HCs remained empty. By contrast, when the retina was incubated at 37°C for one hour in the presence of GABA synthesis precursors, in either aCSF or Ames, HCs appeared loaded with GABA.
We have found that mouse horizontal cells cannot take up GABA but they can synthesize it. The GABA content of HCs is quickly lost during dissection and fixation. HCs are devoid of a GABA uptake system but can synthetize GABA: a simple incubation step in the presence of precursors (glutamate or glutamine) is sufficient to restore GABA in HCs. This study supports further the existence of a GABA synapse in the outer retina.
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