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W. R. Taylor, M. Lipin; Glycinergic Modulation of Excitation in Direction Selective Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4598.
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© ARVO (1962-2015); The Authors (2016-present)
Directional selective ganglion cells (DSGC) signal the direction of image motion. It is known that GABAA-receptor mediated inhibition most likely originating in starburst amacrine cells (SBACs) is essential for generating directional signals. It is also known that glycine receptors are located on bipolar cell terminals and on SBACs, however, their functional role within the directional circuitry is unclear. The objective of the study is to elucidate the role of glycine receptors in DSGC pathway.
Pigmented rabbits from three to ten weeks of age were used. DSGCs were stimulated using dark moving and stationary bars on a light background. Whole-cell patch clamp recordings were made of light-evoked currents in DSGCs at different holding potentials to isolate excitatory and inhibitory currents.
Application of 1 µM strychnine, a selective glycine receptor antagonist, increased the light-evoked fast transient excitatory currents in DSGCs, but decreased a sustained component of the light-evoked OFF-excitatory and inhibitory currents. Increasing the concentration of strychnine to 10 µM further decreased the sustained ON- and OFF- excitatory currents, and almost abolished ON- and OFF- inhibitory currents. These effects were not due to non-specific antagonism of bungarotoxin-sensitive nicotinic receptors, because application of 300 nM methyllycaconitine (MLA), a nicotinic receptor antagonist, increased rather than decreased inhibitory currents, and slightly decreased excitatory currents. Furthermore, 1 µM strychnine decreased OFF- excitatory and inhibitory currents in the presence of MLA. The strychnine-induced inhibition of light-evoked excitatory currents persisted in the presence of SR95531 suggesting that GABAA receptors were not involved.
Since glycine receptors are not present on DSGCs, the findings indicate that glycinergic inhibition within the presynaptic circuitry is important for modulating the excitatory and inhibitory inputs to DSGCs. The selective effects of glycine antagonists on a sustained component of the inputs indicates that glycinergic pathways might be mediate temporal tuning of the inputs.
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