Xin D, Bloomfield S A
Department of Ophthalmology, New York University School of Medicine, New York 10016, USA.
J Comp Neurol. 1999 Mar 1;405(1):75-87. doi: 10.1002/(sici)1096-9861(19990301)405:1<75::aid-cne6>3.0.co;2-d.
Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level.
视网膜水平细胞通过缝隙连接形成广泛的电耦合，从而具有大的感受野。这些缝隙连接的电导似乎受多巴胺通过cAMP介导的级联反应调节。现在有大量证据表明，细胞外多巴胺水平随环境光强度的变化而变化，这表明视网膜暗/亮适应状态的改变可调节水平细胞之间的耦合。我们通过确定以改变背景光强度形式的环境光水平对兔A 型和B型水平细胞耦合情况的影响，来研究哺乳动物视网膜中的这个问题。通过测量水平细胞形成的合体的空间常数以及生物素化示踪剂神经生物素的细胞间扩散来评估耦合的变化。我们的结果表明，暗适应的水平细胞显示出相对较弱的耦合。然而，呈现高于视杆阈值四分之一个对数单位的暗光背景，会导致A 型和B型水平细胞的示踪剂耦合平均范围和空间常数均增加。随着背景光强度增加1 - 1.5个对数单位，耦合进一步扩大，之后进一步的光适应导致细胞解耦。在高于视杆阈值约3个对数单位的光强度下，耦合达到最小值，此后随着进一步的光适应，它稳定在接近暗适应视网膜中所见的水平。我们的结果表明，哺乳动物水平细胞之间的电耦合受视网膜适应状态变化的显著调节：在昏暗的环境光条件下耦合最大化，并且当视网膜从此水平暗适应或明适应时耦合减弱。
