Gustafson Eric G, Stevens Eric S, Miller Robert F
Department of Neuroscience, University of Minnesota Medical School, Minneapolis, MN, 55455, USA.
J Physiol. 2015 Feb 15;593(4):843-56. doi: 10.1113/jphysiol.2014.283432. Epub 2015 Jan 7.
Activation of NMDA receptors (NMDARs) is essential for encoding visual stimuli into signals for the brain, although their over-activation can cause cell death. The recruitment of NMDARs is important for encoding light intensity in retinal ganglion cells. D-serine binding is essential for proper activation of NMDARs, although its role in signal processing and the mechanisms that underlie its availability are not well understood. In these light-evoked experiments, the addition of exogenous D-serine had a large effect on low contrast and low intensity NMDAR responses that decreased as the intensity was increased. The degradation of endogenous D-serine decreased the responses more at higher intensities. The results provide compelling evidence favouring a new interpretation of NMDAR recruitment in which light-evoked D-serine release serves an important regulatory control over the recruitment of NMDARs.
The present study aimed to investigate the functional properties of NMDA receptor coagonist release and to specifically evaluate whether light-evoked release mechanisms contribute to the availability of the coagonist D-serine. Two different methods were involved in our approach: (i) whole-cell recordings from identified retinal ganglion cells in the tiger salamander were used to study light adaptation with positive and negative contrast stimuli over a range of ± 1 log unit against a steady background illumination and (ii) the mechanisms for intensity encoding to a range of light intensities covering 6 log10 units were investigated. This latter study employed extracellular recordings of the proximal negative field potential, pharmacologically manipulated to generate a pure NMDA mediated response. For the adaptation study, we examined the light-evoked responses under control conditions, followed by light stimuli presented in the presence of D-serine, followed by light stimulation in the presence of dichlorokynurenic acid to block the coagonist site of NMDA receptors. For the brightness encoding studies, we examined the action of D-serine on each intensity used and then applied the enzyme D-serine deaminase to remove significant levels of D-serine. These studies provided new insights into the mechanisms that regulate coagonist availability in the vertebrate retina. Our results strongly support the idea that light-evoked coagonist release, a major component of which is D-serine, is needed to provide the full range of coagonist availability for optimal activation of NMDA receptors.
N-甲基-D-天冬氨酸受体（NMDARs）的激活对于将视觉刺激编码为大脑信号至关重要，尽管其过度激活会导致细胞死亡。NMDARs的募集对于视网膜神经节细胞中光强度的编码很重要。D-丝氨酸结合对于NMDARs的正常激活至关重要，尽管其在信号处理中的作用以及其可用性的潜在机制尚不清楚。在这些光诱发实验中，添加外源性D-丝氨酸对低对比度和低强度的NMDAR反应有很大影响，随着强度增加，这种影响会减小。内源性D-丝氨酸的降解在较高强度下对反应的降低作用更大。这些结果提供了令人信服的证据，支持对NMDAR募集的一种新解释，即光诱发的D-丝氨酸释放对NMDARs的募集起着重要的调节控制作用。
本研究旨在研究N-甲基-D-天冬氨酸受体共激动剂释放的功能特性，并具体评估光诱发释放机制是否有助于共激动剂D-丝氨酸的可用性。我们的方法涉及两种不同的方法：（i）使用来自虎螈中已识别的视网膜神经节细胞的全细胞记录，在稳定的背景光照下，用正负对比度刺激在±1对数单位范围内研究光适应；（ii）研究强度编码到覆盖6个对数10单位的一系列光强度的机制。后一项研究采用近端负场电位的细胞外记录，通过药理学方法进行操作以产生纯NMDAR介导的反应。对于适应性研究，我们在对照条件下检查光诱发反应，然后在存在D-丝氨酸的情况下进行光刺激，接着在存在二氯喹啉酸的情况下进行光刺激以阻断NMDARs的共激动剂位点。对于亮度编码研究，我们检查了D-丝氨酸对每个使用强度的作用，然后应用D-丝氨酸脱氨酶去除大量的D-丝氨酸。这些研究为脊椎动物视网膜中共激动剂可用性的调节机制提供了新的见解。我们的结果有力地支持了这样一种观点，即光诱发的共激动剂释放（其主要成分是D-丝氨酸）是为NMDARs的最佳激活提供全范围共激动剂可用性所必需的。
