Light Adaptation in Vertebrate Rod Photoreceptors

描述(申请人提供)：杆状感光器接近终极 但也能够将敏感度调节到 广泛的环境照明条件。这种光适应的过程 是很重要的，因为它扩展了杆的动态范围 对视觉信息进行编码。这些研究的目标是了解 视杆细胞感光敏感度变化的分子基础 放在有稳定照明的地方。光适应是已知的 涉及Ca+2反馈到光转导级联加速视紫红质 关闭，增加cGMP产量，并在较低水平开放离子通道 CGMP。至少其中一些反馈机制运行迅速，并导致 对光步的反应部分恢复或下垂。这个下垂是 基本在几秒钟内完成，并有助于防止杆子饱和 这样它就可以继续发出光强度变化的信号。从吸力 在两栖动物单杆上的电极记录，我们已经获得了证据 光适应的一个组成部分在较慢的时间内起作用 比例。慢成分减敏的时间进程和程度 将会被测量。慢成分对Ca+2反馈的依赖性将 要下定决心。然后将测试三种可能的机制：转导蛋白 随着cGMP从PDE上的非催化位置解离，寿命缩短， 转导蛋白的可获得性降低，因为光导蛋白阻止了它的循环， CGMP门控通道在较低浓度的cGMP下开放，这是由于其 对cGMP的表观亲和力。这三种机制并不是相互排斥的。 有趣的是，两栖类杆状动物对光的适应能力远远超过 哺乳动物的杆状物。本文将探讨两个假设。第一，生化方法 将被用来测试光过程中转导蛋白寿命的缩短 CGMP从非催化状态解离在两栖动物杆中的适应性 在哺乳动物的视杆细胞中，由于cGMP结合，磷酸二酯酶上的位点无法出现。 更紧密地靠近哺乳动物PDE上的这些位置。第二，罕见的，异常的， 长时间的单光子反应发生在哺乳动物的视杆中，但还没有。 出现在两栖动物的鱼竿上。异常反应的累积效应 暴露在光的台阶下可能会使哺乳动物的杆状突起饱和 为时过早。将使用吸电极记录来评估 野生型小鼠杆体和杆状体内对光适应的异常反应 异常反应的频率高于正常。反常的 在某些人类视网膜疾病中，反应发生的频率更高 严格限制暗视的强度范围。

DESCRIPTION (provided by applicant): Rod photoreceptors approach the ultimate in sensitivity to light but are also capable of adjusting that sensitivity over a wide range of ambient lighting conditions. This process of light adaptation is important because it extends the dynamic range over which rods are capable of encoding visual information. The goal of these studies is to understand the molecular bases for the changes in rod photoreceptor sensitivity that take place in the presence of steady illumination. Light adaptation is known to involve Ca+2 feedback onto the phototransduction cascade accelerating rhodopsin shutoff, increasing cGMP production and opening ion channels at lower levels of cGMP. At least some of these feedback mechanisms operate rapidly and cause the response to steps of light to partially recover or droop. This droop is essentially complete in a few seconds and helps to keep the rod from saturating so that it can continue to signal changes in light intensity. From suction electrode recordings on single rods of amphibians, we have obtained evidence that there is a component of light adaptation that operates on a slower time scale. The time course and magnitude of desensitization of the slow component will be measured. The dependency of the slow component upon Ca+2 feedback will be determined. Then three putative mechanisms will be tested: transducin's lifetime shortens as cGMP dissociates from noncatalytic sites on PDE, transducin availability decreases because phosducin prevents its recycling, the cGMP-gated channel opens at lower concentrations of cGMP due to a change in its apparent affinity for cGMP. The three mechanisms are not mutually exclusive. Interestingly, light adaptation in amphibian rods far surpasses that in mammalian rods. Two hypotheses will be explored. First, biochemical methods will be used to test whether the decrease in transducin's lifetime during light adaptation in amphibian rods due to the dissociation of cGMP from noncatalytic sites on phosphodiesterase fails to occur in mammalian rods, because cGMP binds much more tightly to these sites on mammalian PDE. Second, rare, aberrant, prolonged single photon responses occur in mammalian rods, but have not been seen in amphibian rods. The cumulative effects of aberrant responses during exposures to steps of light may drive mammalian rods into saturation prematurely. Suction electrode recording will be used to assess the impact of aberrant responses on light adaptation in wild type mouse rods and in rods where the frequency of aberrant responses is higher than normal. Aberrant responses occur with higher frequency in some human retinal diseases and severely limit the intensity range of scotopic vision.