GABAA, GABAC and glycine receptor‐mediated inhibition differentially affects light‐evoked signalling from mouse retinal rod bipolar cells

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Abstract
Rod bipolar cells relay visual signals evoked by dim illumination from the outer to the inner retina. GABAergic and glycinergic amacrine cells contact rod bipolar cell terminals, where they modulate transmitter release and contribute to the receptive field properties of third order neurones. However, it is not known how these distinct inhibitory inputs affect rod bipolar cell output and subsequent retinal processing. To determine whether GABAA, GABAC and glycine receptors made different contributions to light-evoked inhibition, we recorded light-evoked inhibitory postsynaptic currents (L-IPSCs) from rod bipolar cells mediated by each pharmacologically isolated receptor. All three receptors contributed to L-IPSCs, but their relative roles differed; GABAC receptors transferred significantly more charge than GABAA and glycine receptors. We determined how these distinct inhibitory inputs affected rod bipolar cell output by recording light-evoked excitatory postsynaptic currents (L-EPSCs) from postsynaptic AII and A17 amacrine cells. Consistent with their relative contributions to L-IPSCs, GABAC receptor activation most effectively reduced the L-EPSCs, while glycine and GABAA receptor activation reduced the L-EPSCs to a lesser extent. We also found that GABAergic L-IPSCs in rod bipolar cells were limited by GABAA receptor-mediated inhibition between amacrine cells. We show that GABAA, GABAC and glycine receptors mediate functionally distinct inhibition to rod bipolar cells, which differentially modulated light-evoked rod bipolar cell output. Our findings suggest that modulating the relative proportions of these inhibitory inputs could change the characteristics of rod bipolar cell output.