Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3− efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

光感受器向二级双极细胞传递的视觉信息流，会被侧向相互作用的水平细胞所截留；此类水平细胞可生成反馈信号，以优化并提升信号转导效率。
在该非尖峰神经网络中，调控分级式光感受器突触输出的潜在机制长期以来仍未阐明。
本研究借助膜片钳记录（patch clamp recording）技术，分析了水平细胞通过调控突触间隙pH值，以调节光感受器神经递质释放的全新机制。
首先，我们证实哺乳动物水平细胞可响应自身释放的γ-氨基丁酸（GABA），且该自突触作用可通过改变pH值，影响视锥细胞的电压门控Ca2+通道（voltage-gated Ca2+ channel, CaV通道）门控过程。
作为原理验证实验，我们证明通过外源性阴离子通道表达对水平细胞进行化学遗传学操控，可模拟GABA介导的视锥细胞CaV通道抑制效应。
此类γ-氨基丁酸受体（GABA receptor, GABAR）阴离子通道的激活，可使水平细胞发生去极化，并通过钠氢交换体（Na+/H+ exchanger, NHE）的质子外排作用升高突触间隙的酸度，进而抑制视锥细胞的CaV通道。
当水平细胞因GABAR介导的碳酸氢根（HCO3−）外流增加而充分超极化时，上述作用会被有效抵消，使得突触间隙碱化并解除对视锥细胞CaV通道的抑制。
该研究揭示了GABA的复合作用如何并行调控电压依赖性pH变化，这是一种全新的突触输出调控机制。