Rohrbough J, Spitzer N C
Department of Biology, University of California at San Diego, La Jolla 92093-0357, USA.
J Neurosci. 1996 Jan;16(1):82-91. doi: 10.1523/JNEUROSCI.16-01-00082.1996.
Rohon-Beard (RB) spinal neurons of Xenopus larvae are depolarized by GABA. To study the mechanisms underlying this distinctive response, intracellular and patch-clamp recordings were made from RB neurons in situ. The intracellularly recorded GABA reversal potential (EREV) was near -30 mV in normal saline and was approximately 25 mV more negative in Na(+)-free saline. Whole-cell recordings from RB neurons and from neighboring dorsolateral interneurons (DLi) revealed that GABA responses of both cells were mediated by GABAA receptors. Currents elicited by GABA were mimicked by muscimol and reversibly blocked by bicuculline, and EREV shifted with changes in Cl- concentration ([Cl]) in agreement with Cl- selectivity. In perforated patch recordings, EREV for RB cells was significantly more positive than for DLi cells (-38 vs -63 mV), indicating that intact RB cells maintain higher levels of intracellular Cl-. Replacement of external Na+ or exposure to the Cl- transport inhibitor bumetanide (100 microM) shifted RB cell EREV to move negative values, consistent with Na+(-)dependent Cl cotransport contributing to higher internal [Cl]. In contrast, these treatments did not change DLi cell EREV. The results indicate that a Na+(-)dependent Cl- transport mechanism underlies GABAA receptor-mediated depolarizing Cl- conductances in RB neurons. Thus, both inhibitory and excitatory GABA responses appear to be present during the same developmental period in vivo. GABA may stimulate Ca2+ influx in RB neurons because the intracellular GABA EREV is above the threshold for low voltage-activated Ca2+ channels.
非洲爪蟾幼体的罗霍恩-比尔（RB）脊髓神经元会被γ-氨基丁酸（GABA）去极化。为了研究这种独特反应背后的机制，对原位的RB神经元进行了细胞内和膜片钳记录。在正常盐溶液中，细胞内记录的GABA反转电位（EREV）接近-30 mV，而在无钠盐溶液中则负约25 mV。对RB神经元和相邻的背外侧中间神经元（DLi）进行全细胞记录显示，两种细胞的GABA反应均由GABAA受体介导。GABA引发的电流可被蝇蕈醇模拟，并被荷包牡丹碱可逆性阻断，且EREV随氯离子浓度（[Cl]）的变化而移动，符合氯离子选择性。在穿孔膜片记录中，RB细胞的EREV明显比DLi细胞更正（-38 mV对-63 mV），表明完整的RB细胞维持较高水平的细胞内氯离子。外部钠离子的置换或暴露于氯离子转运抑制剂布美他尼（100 μM）会使RB细胞的EREV向负值移动，这与钠离子依赖的氯共转运导致细胞内[Cl]升高一致。相比之下，这些处理并未改变DLi细胞的EREV。结果表明，钠离子依赖的氯转运机制是RB神经元中GABAA受体介导的去极化氯离子电导的基础。因此，在体内同一发育时期似乎同时存在抑制性和兴奋性GABA反应。GABA可能会刺激RB神经元中的钙离子内流，因为细胞内GABA的EREV高于低电压激活钙离子通道的阈值。
