Gao F, Maple B R, Wu S M
Cullen Eye Institute, Baylor College of Medicine, Houston, Texas 77030, USA.
J Neurophysiol. 2000 Jun;83(6):3473-82. doi: 10.1152/jn.2000.83.6.3473.
Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: DeltaI(C), a glutamate-gated cation current with a reversal potential near 0 mV, and DeltaI(Cl), a chloride current with a reversal potential near -60 mV. In DBCs DeltaI(C) was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs DeltaI(Cl) was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA(C) receptor antagonist. In all DBCs and HBCs examined, 10 microM I4AA eliminated DeltaI(Cl) and the light-evoked current became predominately mediated by DeltaI(C). The addition of 20 microM L-AP4 to the DBCs or 50 microM DNQX to HBCs completely abolished DeltaI(C). Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 microM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (DeltaI(Cl)) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA(C) receptors on bipolar cell axon terminals. Picrotoxin (200 microM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA(C) receptor antagonists) did not block (but enhanced and broadened) the light-evoked DeltaI(Cl), although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA(C) receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked DeltaI(Cl) in bipolar cells are mediated by the more strongly I4AA-sensitive GABA(C) receptors. Picrotoxin, although acting as a partial GABA(C) receptor antagonist in bipolar cells, does not suppress DeltaI(Cl) because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.
在虎螈幼体的活体视网膜切片上，在电压钳制条件下记录了去极化双极细胞（DBCs）和超极化双极细胞（HBCs）中的光诱发电流。DBCs和HBCs感受野中心对光照的反应由两种突触后电流介导：DeltaI(C)，一种谷氨酸门控阳离子电流，其反转电位接近0 mV；以及DeltaI(Cl)，一种氯离子电流，其反转电位接近-60 mV。在DBCs中，DeltaI(C)被L-2-氨基-4-膦酰丁酸（L-AP4）抑制，而在HBCs中，它被6,7-二硝基喹喔啉-2,3-二酮（DNQX）抑制。在DBCs和HBCs中，DeltaI(Cl)均被咪唑-4-乙酸（I4AA）抑制，I4AA是一种GABA受体激动剂和GABA(C)受体拮抗剂。在所有检测的DBCs和HBCs中，10 microM的I4AA消除了DeltaI(Cl)，光诱发电流主要由DeltaI(C)介导。向DBCs中添加20 microM的L-AP4或向HBCs中添加50 microM的DNQX完全消除了DeltaI(C)。在内网状层局部施加谷氨酸会使双极细胞产生氯离子电流，这是通过使无长突细胞去极化实现的，无长突细胞在双极细胞轴突终末的突触处释放GABA，而DBCs和HBCs中这种谷氨酸诱导产生的氯离子电流可被10 microM的I4AA可逆性阻断。这些实验表明，DBCs和HBCs中光诱发的、I4AA敏感的氯离子电流（DeltaI(Cl)）是由窄场GABA能无长突细胞介导的，这些无长突细胞激活双极细胞轴突终末上的GABA(C)受体。印防己毒素（200 microM）或（1,2,5,6-四氢吡啶-4基）甲基次膦酸（TPMPA）（另外两种GABA(C)受体拮抗剂）并未阻断（但增强并拓宽了）光诱发的DeltaI(Cl)，尽管它们降低了通过微量注射GABA或谷氨酸诱导产生的氯离子电流。窄场无长突细胞的光反应不受I4AA影响，但被印防己毒素显著增强并拓宽。这些结果表明，双极细胞中至少存在两种类型的GABA(C)受体：一种对I4AA的敏感性比另一种更强，但二者均可被印防己毒素部分阻断。窄场无长突细胞中的GABA受体对I4AA不敏感，对印防己毒素敏感。双极细胞中光诱发的DeltaI(Cl)由对I4AA敏感性更强的GABA(C)受体介导。印防己毒素虽然在双极细胞中作为部分GABA(C)受体拮抗剂起作用，但并不抑制DeltaI(Cl)，因为其对无长突细胞光反应的突触前效应超过了其突触后拮抗作用。
