Research Unit Retinal Signal Processing, The Netherlands Institute for Neuroscience, Amsterdam, The Netherlands.
PLoS Biol. 2011 Jul;9(7):e1001107. doi: 10.1371/journal.pbio.1001107. Epub 2011 Jul 19.
In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.
在脊椎动物的视网膜中，水平细胞产生双极细胞的抑制环绕，这是对比度增强的一个重要步骤。在过去的几十年里，这个抑制性突触通路涉及的机制一直是视网膜研究中的一个主要争议。一种假设认为连接蛋白半通道介导了这种负反馈信号；另一种假设认为反馈是由质子介导的。生成了缺乏水平细胞中连接蛋白 55.5 半通道的突变斑马鱼。从分离的水平细胞和平面安装视网膜中的锥体进行全细胞膜电压钳记录。在突变体中，水平细胞对锥体的光诱导反馈减少。当水平细胞被药理学去极化时，也发现了反馈的减少，但当它们被药理学超极化时，反馈就不存在了。分离的水平细胞中的半通道电流也表现出类似的行为。在突变体中，去极化诱导的半通道电流没有减少，而超极化诱导的半通道电流则显著减少。从水平细胞进行细胞内记录。与突变体中反馈受损一致，这些动物中水平细胞的光谱对立反应减弱。行为测定显示，对比度敏感性降低，说明了水平细胞到锥体反馈通路在对比度增强中的作用。模型模拟表明，观察到的反馈的修饰可以用电突触机制来解释。一种反馈模型，其中连接蛋白半通道的数量减少到约 40%，完全预测了反馈的特定不对称修饰。据我们所知，这是在水平细胞到锥体的反馈通路中首次成功的遗传干扰。它为连接蛋白半通道在水平细胞和锥体之间的抑制性突触中的非传统作用提供了直接证据。这是解决脊椎动物视网膜中水平细胞和锥体之间的（电突触）突触传递的不寻常形式的长期争论的重要一步。
