Light Adaptation in Vertebrate Rod Photoreceptors

描述（由申请人提供）：视杆细胞接近极限 对光的灵敏度有影响，但也能够调整灵敏度 广泛的环境照明条件。这种适应光线的过程 是重要的，因为它扩展了杆能够 编码视觉信息。这些研究的目的是了解 视杆细胞感光器敏感性变化的分子基础 在稳定的照明下放置。已知光适应 涉及Ca+2反馈到加速视紫红质的光转导级联 关闭，增加cGMP的生产和开放离子通道在较低的水平， cGMP。这些反馈机制中的至少一些快速地操作并且引起反馈机制的改变。 响应光的步骤，以部分恢复或下降。这种下垂是 基本上在几秒钟内完成，并有助于保持杆从饱和 这样它就能持续发出光强度变化的信号。从吸入 电极记录两栖动物的单杆，我们已经获得了证据， 有一种光适应的成分， 规模慢成分减敏的时间过程和幅度 将被衡量。慢成分对Ca+2反馈的依赖性将 被确定。然后将测试三种假定的机制： 由于cGMP从PDE上的非催化位点解离， transducin的可用性降低，因为transducin阻止其再循环， cGMP门控通道在较低浓度的cGMP下打开，这是由于其 对cGMP的表观亲和力。这三种机制并不相互排斥。 有趣的是，两栖类杆状动物的光适应远远超过了 哺乳动物的视杆细胞将探讨两个假设。第一，生化方法 将用于测试光下转导蛋白寿命的减少是否 两栖类视杆细胞由于cGMP从非催化 磷酸二酯酶上的位点不能出现在哺乳动物的视杆细胞中，因为cGMP结合 与哺乳动物PDE上的这些位点紧密得多。第二，罕见，异常， 在哺乳动物的视杆细胞中发生了延长的单光子反应，但还没有 在两栖动物的竿上发现的异常反应的累积效应， 暴露在光的阶梯下可能会驱使哺乳动物的视杆细胞进入饱和状态 过早地。吸引电极记录将用于评估 野生型小鼠视杆细胞和视杆细胞对光适应的异常反应 其中异常反应的频率高于正常。异常 在某些人类视网膜疾病中， 严重限制了暗视觉的强度范围。

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.