Han Bingjie, Bellemer Andrew, Koelle Michael R
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520.
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520
Genetics. 2015 Apr;199(4):1159-72. doi: 10.1534/genetics.114.173963. Epub 2015 Feb 2.
The neurotransmitter gamma-aminobutyric acid (GABA) is depolarizing in the developing vertebrate brain, but in older animals switches to hyperpolarizing and becomes the major inhibitory neurotransmitter in adults. We discovered a similar developmental switch in GABA response in Caenorhabditis elegans and have genetically analyzed its mechanism and function in a well-defined circuit. Worm GABA neurons innervate body wall muscles to control locomotion. Activation of GABAA receptors with their agonist muscimol in newly hatched first larval (L1) stage animals excites muscle contraction and thus is depolarizing. At the mid-L1 stage, as the GABAergic neurons rewire onto their mature muscle targets, muscimol shifts to relaxing muscles and thus has switched to hyperpolarizing. This muscimol response switch depends on chloride transporters in the muscles analogous to those that control GABA response in mammalian neurons: the chloride accumulator sodium-potassium-chloride-cotransporter-1 (NKCC-1) is required for the early depolarizing muscimol response, while the two chloride extruders potassium-chloride-cotransporter-2 (KCC-2) and anion-bicarbonate-transporter-1 (ABTS-1) are required for the later hyperpolarizing response. Using mutations that disrupt GABA signaling, we found that neural circuit development still proceeds to completion but with an ∼6-hr delay. Using optogenetic activation of GABAergic neurons, we found that endogenous GABAA signaling in early L1 animals, although presumably depolarizing, does not cause an excitatory response. Thus a developmental depolarizing-to-hyperpolarizing shift is an ancient conserved feature of GABA signaling, but existing theories for why this shift occurs appear inadequate to explain its function upon rigorous genetic analysis of a well-defined neural circuit.
神经递质γ-氨基丁酸（GABA）在发育中的脊椎动物大脑中会引起去极化，但在成年动物中则转变为超极化，并成为主要的抑制性神经递质。我们在秀丽隐杆线虫中发现了GABA反应中类似的发育转变，并对其在一个明确界定的神经回路中的机制和功能进行了遗传学分析。线虫的GABA能神经元支配体壁肌肉以控制运动。在刚孵化的第一期幼虫（L1）阶段的动物中，用其激动剂蝇蕈醇激活GABAA受体会激发肌肉收缩，因此是去极化的。在L1中期，随着GABA能神经元重新连接到其成熟的肌肉靶标上，蝇蕈醇转变为使肌肉松弛，因此已转变为超极化。这种蝇蕈醇反应转变取决于肌肉中的氯离子转运体，类似于那些控制哺乳动物神经元中GABA反应的转运体：氯离子蓄积体钠-钾-氯协同转运体-1（NKCC-1）是早期去极化的蝇蕈醇反应所必需的，而两种氯离子排出体钾-氯协同转运体-2（KCC-2）和阴离子-碳酸氢盐转运体-1（ABTS-1）是后期超极化反应所必需的。利用破坏GABA信号传导的突变，我们发现神经回路发育仍能继续完成，但会延迟约6小时。利用对GABA能神经元的光遗传学激活，我们发现L1早期动物中的内源性GABAA信号传导，尽管推测是去极化的，但不会引起兴奋性反应。因此，从去极化到超极化的发育转变是GABA信号传导的一个古老保守特征，但关于这种转变为何发生的现有理论，在对一个明确界定的神经回路进行严格的遗传学分析时，似乎不足以解释其功能。
