| Voltage-gated currents and non-NMDA-, NMDA-, glycine-, and GABA-gated currents on All amacrine and cone bipolar cells were examined, using whole-cell recordings in rabbit retina slice preparations superfused with a Co 2+ containing Ringer's solution to block synaptic transmission. The AII amacrine cells displayed a distinct narrow-field, bistratified morphology. Voltage-gated sodium currents fast activating and slowly inactivating were found. Potassium currents resembling a delayed-rectifier type or a variation of A-type currents were expressed. Kainate-evoked currents were abolished by CNQX, GYK153655, and SYM2206. Applications of SYM2081 or AP-4 puffs elicited no currents, suggesting that only AMPA receptors were expressed, not kainate or mGluR receptors on AII amacrine cells. After Co2+ ions were removed from Ringer's solutions, NMDA-evoked currents blocked by D-AP7 were found and attenuated by the presence of Co2+, Mg2+, and Ca2+ ions. GABA-activated currents were almost blocked by an application of bicuculline, not TPMPA. Glycine-activated currents were also found, suggesting the presence of glycine autoreceptors on All amacrine cells. The CBa1, CBa2, and a small proportion of CBa1-2 off-cone bipolar cells had demonstrated larger glycine-activated currents than GABA-activated currents on axon terminals. The CBa2n, CBa1-2n, CBb3, CBb3-4, CBb3n, and CBb4 cells had higher GABA-activated currents than glycine, while CBb5 had only GABA-activated currents. This suggested that CBa1, CBa2, and a small proportion of CBa1-2 cells played a major role in the rod pathway. Off-cone bipolar cells appeared to have almost equal GABAA and GABAC currents. However, CBb3, CBb3-4, and CBb3" had more GABAC than GABAA currents, and CBb4 and CBb5 had more GABAA than GABAC currents. On the dendrites of CBa1, CBa2, and CBa2 off-cone bipolar cells, major AMPA plus minor KA receptors mediated kainate-evoked currents; while on the dendrites of CBa2n and CBa1-2n off-cone bipolar cells, only AMPA receptors were expressed. No kainate-evoked currents were found on rod bipolar cells and on-cone bipolar cells, suggesting that major AMPA plus minor KA receptors were expressed on the dendrites of off-cone bipolar cells, not on-cone bipolar cells. On the dendrites of CBa1, CBa1-2, and CBa2n off-cone bipolar cells, AMPA receptors or KA receptors displayed an inward rectifier property with a high calcium permeability, suggesting presumably different subunit compositions. |
