Cunningham J R, Neal M J
J Physiol. 1985 May;362:51-67. doi: 10.1113/jphysiol.1985.sp015662.
The effects of excitatory amino acids, analogues and K on [3H]gamma-aminobutyric acid [3H]GABA) release from horizontal cells of the isolated superfused frog retina were studied. Exposure of the retina to medium containing high concentrations (25-100 mM) of KCl increased the release of [3H]GABA to a maximum which was 40 times the spontaneous resting release. The K-evoked release of [3H]GABA was almost abolished in high-Mg/low-Ca medium. Glutamate, aspartate, kainate and quisqualate also stimulated the release of [3H]GABA from horizontal cells, the maximum evoked release being similar to that produced by KCl. The release of [3H]GABA evoked by glutamate, aspartate, kainate and quisqualate was abolished in high-Mg/low-Ca medium and by Na-free medium. The evoked releases of [3H]GABA were not reduced by tetrodotoxin. N-Methyl-D-aspartate (NMDA) at concentrations up to 10 mM had virtually no effect on [3H]GABA release from horizontal cells. In Mg-free medium, NMDA stimulated [3H]GABA release, but the maximum release was only 10% of that produced by other agonists. Mg-free medium did not significantly affect the evoked release of [3H]GABA by other agonists. NMDA apparently possessed affinity for the kainate receptor, because in normal medium it antagonized the effects of kainate but not glutamate, aspartate or quisqualate. The non-selective antagonist of excitatory amino acids, (+/-)-cis-2,3-piperidine dicarboxylic acid (PDA) antagonized the action of glutamate, aspartate, kainate and quisqualate on horizontal cell [3H]GABA release. D(-)-2-Amino-4-phosphonobutyrate (APB) and D-gamma-glutamylglycine (D-gamma-GG) antagonized the actions of kainate on horizontal cell [3H]GABA release at concentrations which had little affect on quisqualate-evoked responses. Approximate estimates of pA2 values (Schild, 1947) showed that the specificity and potency of the antagonists was low. Nevertheless, the retinal 'non-NMDA' receptors can probably be subdivided into kainate and quisqualate types. Glutamate diethylester (GDEE) did not affect the action of any agonist. We conclude that glutamate (and aspartate) probably stimulate the release of [3H]GABA from frog horizontal cells by activating receptors of the non-NMDA type. This activation may trigger the opening of tetrodotoxin-insensitive Na channels, resulting in the depolarization of the cell membrane and an increase in the conductance of voltage-sensitive Ca-channels. An influx of Ca ions would then trigger the release of [3H]GABA. Our results are not consistent with previous suggestions that GABA release from horizontal cells involves an outwardly directed transport process.
研究了兴奋性氨基酸、类似物及钾对离体灌流青蛙视网膜水平细胞释放[³H]γ-氨基丁酸（[³H]GABA）的影响。将视网膜暴露于含高浓度（25 - 100 mM）氯化钾的培养基中，可使[³H]GABA的释放增加至最大值，该最大值是静息自发释放的40倍。在高镁/低钙培养基中，钾诱发的[³H]GABA释放几乎完全被抑制。谷氨酸、天冬氨酸、 kainate和quisqualate也能刺激水平细胞释放[³H]GABA，最大诱发释放与氯化钾所产生的相似。在高镁/低钙培养基及无钠培养基中，谷氨酸、天冬氨酸、kainate和quisqualate诱发的[³H]GABA释放被抑制。河豚毒素不降低[³H]GABA的诱发释放。浓度高达10 mM的N-甲基-D-天冬氨酸（NMDA）对水平细胞释放[³H]GABA几乎无影响。在无镁培养基中，NMDA刺激[³H]GABA释放，但最大释放量仅为其他激动剂所产生释放量的10%。无镁培养基对其他激动剂诱发的[³H]GABA释放无显著影响。NMDA显然对kainate受体具有亲和力，因为在正常培养基中它拮抗kainate的作用，但不拮抗谷氨酸、天冬氨酸或quisqualate的作用。兴奋性氨基酸的非选择性拮抗剂（±）-顺式-2,3-哌啶二羧酸（PDA）拮抗谷氨酸、天冬氨酸、kainate和quisqualate对水平细胞[³H]GABA释放的作用。D（-）-2-氨基-4-膦酰丁酸（APB）和D-γ-谷氨酰甘氨酸（D-γ-GG）在对quisqualate诱发反应影响较小的浓度下，拮抗kainate对水平细胞[³H]GABA释放的作用。对pA2值（Schild，1947）的近似估计表明，拮抗剂的特异性和效能较低。然而，视网膜“非NMDA”受体可能可细分为kainate型和quisqualate型。谷氨酸二乙酯（GDEE）不影响任何激动剂的作用。我们得出结论，谷氨酸（和天冬氨酸）可能通过激活非NMDA型受体刺激青蛙水平细胞释放[³H]GABA。这种激活可能触发对河豚毒素不敏感的钠通道开放，导致细胞膜去极化并增加电压敏感性钙通道的电导。钙离子内流随后会触发[³H]GABA的释放。我们的结果与先前关于水平细胞释放GABA涉及外向转运过程的观点不一致。
