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W Shen, MM Slaughter; The Glycinergic Interplexiform Cell Modulates The Photoreceptor Synapse . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2923.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The interplexiform cell (IPC) has remained the least defined of retinal neurons. It feeds back from the inner to the outer retina, but its function is unclear. The amphibian retina offers a valuable preparation for evaluating the role of these neurons because the IPC is the only glycinergic neuron in the outer plexiform layer. Therefore, we examined the effect of glycine and glycine antagonists on the regulation of the photoreceptor to horizontal cell synapse. Methods: Whole cell patch-clamp recording was performed in the tiger salamander retina slice, using both current-clamp and voltage-clamp protocols. Drugs were applied by a rapid superfusion system while red and green LED stimuli were used to preferentially stimulate cones and rods, respectively. Results: Glycine at concentrations between 20 and 200 µM depolarized horizontal cells, but the effect of low concentrations of glycine was eliminated by cobalt (which blocks calcium-dependent transmitter release) and by CNQX (which blocks glutamate receptors) while the effect of high concentrations of glycine persisted in the presence of these two agents. This indicates that high concentrations of glycine act directly on the horizontal cell, while low concentrations work indirectly by increasing glutamatergic synaptic transmission. The action of high glycine concentrations was fast, but that of lower concentrations was slow. Both effects of glycine were blocked by strychnine. In the dark-adapted retinal slice, the horizontal cell dark membrane potential was very hyperpolarized (about -50 mV). Low concentrations of glycine depolarized the dark membrane potential, and increased the amplitude of the light response. Strychnine alone produced the opposite response, hyperpolarizing the dark membrane potential and reducing the light response of the horizontal cell. In the light-adapted retina, the dark membrane potential of the horizontal cell is more positive (about -25 mV) and the effect of low concentrations of glycine is greatly reduced. Conclusions: The glycinergic IPC enhances photoreceptor to horizontal cell synaptic communication by a presynaptic mechanism. This modulation activates slowly and turns off slowly. Thus, there is a positive feedback system from the glycinergic IPC to the photoreceptors, which is prominent in the dark-adapted retina.
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