Gaspary H L, Wang W, Richerson G B
Department of Neurology, Veteran's Affairs Medical Center, West Haven, Connecticut 06510, USA.
J Neurophysiol. 1998 Jul;80(1):270-81. doi: 10.1152/jn.1998.80.1.270.
gamma-Aminobutyric acid (GABA) transporters are electrogenic and sodium-dependent and can operate in reverse when cells are depolarized or when there is reversal of the inward sodium gradient. However, the functional relevance of this phenomenon is unclear. We have examined whether depolarization induced by a physiologically relevant increase in extracellular [K+] leads to sufficient amounts of carrier-mediated GABA release to activate GABAA receptors on neurons. Patch-clamp recordings were made from rat hippocampal neurons in culture with solutions designed to isolate chloride currents in the recorded neuron. Pressure microejection was used to increase extracellular [K+] from 3 to 12 mM. After blockade of vesicular GABA release by removal of extracellular calcium, this stimulus induced a large conductance increase in hippocampal neurons [18.9 +/- 6.8 (SD) nS; n = 16]. This was blocked by the GABAA receptor antagonists picrotoxin and bicuculline and had a reversal potential that followed the Nernst potential for chloride, indicating that it was mediated by GABAA receptor activation. Similar responses occurred after block of vesicular neurotransmitter release by tetanus toxin. GABAA receptors also were activated when an increase in extracellular [K+] (from 3 to 13 mM) was combined with a reduction in extracellular [Na+] or when cells were exposed to a decrease in extracellular [Na+] alone. These results indicate that depolarization and/or reversal of the Na+ gradient activated GABA receptors via release of GABA from neighboring cells. We found that the GABA transporter antagonists 1-(4, 4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF89976A; 20-100 microM) and 1-(2-([(diphenylmethylene)amino]oxy)ethyl) -1, 2, 5, 6 - tetrahydro - 3 - pyridine - carboxylic acid hydrochloride (NO-711; 10 microM) both decreased the responses, indicating that the release of GABA resulted from reversal of the GABA transporter. We propose that carrier-mediated GABA release occurs in vivo during high-frequency neuronal firing and seizures, and dynamically modulates inhibitory tone.
γ-氨基丁酸（GABA）转运体是电生性且依赖钠的，当细胞去极化或内向钠梯度逆转时，其可以反向运作。然而，这种现象的功能相关性尚不清楚。我们研究了细胞外[K⁺]在生理相关水平升高所诱导的去极化是否会导致足够量的载体介导的GABA释放，从而激活神经元上的GABAA受体。使用旨在分离记录神经元中氯离子电流的溶液，对培养的大鼠海马神经元进行膜片钳记录。通过压力微量注射将细胞外[K⁺]从3 mM增加到12 mM。在去除细胞外钙以阻断囊泡GABA释放后，这种刺激诱导海马神经元的电导大幅增加[18.9±6.8（标准差）nS；n = 16]。这被GABAA受体拮抗剂印防己毒素和荷包牡丹碱阻断，并且其反转电位遵循氯离子的能斯特电位，表明它是由GABAA受体激活介导的。在用破伤风毒素阻断囊泡神经递质释放后也出现了类似的反应。当细胞外[K⁺]升高（从3 mM到13 mM）并伴有细胞外[Na⁺]降低时，或者当细胞单独暴露于细胞外[Na⁺]降低时，GABAA受体也会被激活。这些结果表明，去极化和/或Na⁺梯度的逆转通过相邻细胞释放GABA来激活GABA受体。我们发现GABA转运体拮抗剂1-（4,4-二苯基-3-丁烯基）-3-哌啶羧酸盐酸盐（SKF89976A；20 - 100 μM）和1-（2-（[（二苯基亚甲基）氨基]氧基）乙基）-1,2,5,6-四氢-3-吡啶羧酸盐酸盐（NO-711；10 μM）均降低了反应，表明GABA的释放是由GABA转运体的反向运作导致的。我们提出，在体内高频神经元放电和癫痫发作期间会发生载体介导的GABA释放，并动态调节抑制性张力。
