Picaud S, Larsson H P, Wellis D P, Lecar H, Werblin F
Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA.
Proc Natl Acad Sci U S A. 1995 Sep 26;92(20):9417-21. doi: 10.1073/pnas.92.20.9417.
Pulse-like currents resembling miniature postsynaptic currents were recorded in patch-clamped isolated cones from the tiger salamander retina. The events were absent in isolated cones without synaptic terminals. The frequency of events was increased by either raising the osmotic pressure or depolarizing the cell. It was decreased by the application of either glutamate or the glutamate-transport blockers dihydrokainate and D,L-threo-3-hydroxyaspartate. The events required external Na+ for which Li+ could not substitute. The reversal potential of these currents followed the equilibrium potential for Cl- when internal Cl- concentration was changed. Thus, these miniature currents appear to represent the presynaptic activation of the glutamate receptor with glutamate transporter-like pharmacology, caused by the photoreceptor's own vesicular glutamate release. Using a noninvasive method to preserve the intracellular Cl- concentration, we showed that glutamate elicits an outward current in isolated cones. Fluorescence of the membrane-permeable form of fura-2 was used to monitor Ca2+ entry at the cone terminal as a measure of membrane depolarization. The increase in intracellular Ca2+ concentration, elicited by puff application of 30 mM KCl, was completely suppressed in the presence of 100 microM glutamate. Puff application of glutamate alone had no measurable depolarizing effect. These results suggest that the equilibrium potential for Cl-, ECl, was more negative than the activation range for Ca2+ channels and that glutamate elicited an outward current, hyperpolarizing the cones.
在膜片钳记录的虎螈视网膜分离视锥细胞中，记录到了类似微小突触后电流的脉冲样电流。在没有突触终末的分离视锥细胞中未出现这些事件。通过提高渗透压或使细胞去极化，事件频率增加。而应用谷氨酸或谷氨酸转运体阻滞剂二氢卡因酸和D，L-苏式-3-羟基天冬氨酸可使其频率降低。这些事件需要细胞外Na⁺，Li⁺不能替代。当细胞内Cl⁻浓度改变时，这些电流的反转电位遵循Cl⁻的平衡电位。因此，这些微小电流似乎代表了由光感受器自身囊泡谷氨酸释放引起的、具有类似谷氨酸转运体药理学特性的谷氨酸受体的突触前激活。使用一种无创方法来维持细胞内Cl⁻浓度，我们发现谷氨酸在分离的视锥细胞中引发外向电流。使用膜通透性形式的fura-2荧光来监测视锥细胞终末的Ca²⁺内流，作为膜去极化的指标。在存在100 μM谷氨酸的情况下，由30 mM KCl微量喷射引起的细胞内Ca²⁺浓度增加被完全抑制。单独微量喷射谷氨酸没有可测量的去极化作用。这些结果表明，Cl⁻的平衡电位ECl比Ca²⁺通道的激活范围更负，并且谷氨酸引发外向电流，使视锥细胞超极化。
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