| Retinal bipolar cell | Axon terminal | Gaba | amacrine | The rod-dominant ON-type bipolar cells and some bipolar cells with a small axon terminal receive negative feedback inputs from GABAergic amacrine cells (Tachibana M and Kaneko A, 1988384 ). Depolarization induced transient outward currents that resembled IPSCs and were blocked by GABA and glycine receptor antagonists, suggesting that they arise from activation of amacrine feedback synapses (Protti DA and Llano I, 1998378 ). The OFF cone bipolar cells seem dominated by glycinergic input and the ON cone bipolar and rod bipolar cells by GABAergic input (Grunert U, 2000451 ). The responses of most retinal ganglion cells are transient because bipolar-to-ganglion cell transmission is truncated after 150 msec by a feedback inhibition to bipolar cell terminals from GABAergic amacrine cells; the feedback inhibition itself must be delayed by approximately 150 msec to allow the initial bipolar-ganglion cell transmission.
One source of the delay appears to be glycinergic amacrine cells to GABAergic amacrine cells to bipolar cell terminals. Results suggest that, after a light flash, a population of glycinergic amacrine cells responds first, inhibiting a population of GABAergic amacrine cells for approximately 150 msec. The GABAergic amacrine cells feed back to bipolar terminals, only after the 150 msec delay, thus allowing the bipolar terminals to excite ganglion cells for the first 150 msec. (Roska B and Nemeth E and Werblin FS, 1998452 ). |