Neurosciences Graduate Program, Department of Physiology and Biophysics, USC Keck School of Medicine, Los Angeles, CA, USA.
J Neurophysiol. 2012 May;107(10):2649-59. doi: 10.1152/jn.01202.2011. Epub 2012 Feb 15.
The nervous system frequently integrates parallel streams of information to encode a broad range of stimulus strengths. In mammalian retina it is generally believed that signals generated by rod and cone photoreceptors converge onto cone bipolar cells prior to reaching the retinal output, the ganglion cells. Near absolute visual threshold a specialized mammalian retinal circuit, the rod bipolar pathway, pools signals from many rods and converges on depolarizing (AII) amacrine cells. However, whether subsequent signal flow to OFF ganglion cells requires OFF cone bipolar cells near visual threshold remains unclear. Glycinergic synapses between AII amacrine cells and OFF cone bipolar cells are believed to relay subsequently rod-driven signals to OFF ganglion cells. However, AII amacrine cells also make glycinergic synapses directly with OFF ganglion cells. To determine the route for signal flow near visual threshold, we measured the effect of the glycine receptor antagonist strychnine on response threshold in fully dark-adapted retinal cells. As shown previously, we found that response threshold for OFF ganglion cells was elevated by strychnine. Surprisingly, strychnine did not elevate response threshold in any subclass of OFF cone bipolar cell. Instead, in every OFF cone bipolar subclass strychnine suppressed tonic glycinergic inhibition without altering response threshold. Consistent with this lack of influence of strychnine, we found that the dominant input to OFF cone bipolar cells in darkness was excitatory and the response threshold of the excitatory input varied by subclass. Thus, in the dark-adapted mouse retina, the high absolute sensitivity of OFF ganglion cells cannot be explained by signal transmission through OFF cone bipolar cells.
神经系统经常整合并行信息流以编码广泛的刺激强度。在哺乳动物视网膜中，人们普遍认为，杆状和锥状光感受器产生的信号在到达视网膜输出——神经节细胞之前，先汇聚到锥状双极细胞上。在接近绝对视觉阈值的情况下，哺乳动物视网膜的一个特殊回路——杆状双极通路，会汇集来自许多杆状细胞的信号，并汇聚到去极化（AII）无长突细胞上。然而，在视觉阈值附近，后续信号是否流向 OFF 神经节细胞尚不清楚。AII 无长突细胞与 OFF 锥状双极细胞之间的 Glycinergic 突触被认为是将随后的由杆状细胞驱动的信号传递到 OFF 神经节细胞。然而，AII 无长突细胞也与 OFF 神经节细胞直接形成 Glycinergic 突触。为了确定在视觉阈值附近的信号流的路径，我们测量了甘氨酸受体拮抗剂士的宁对完全暗适应的视网膜细胞的反应阈值的影响。如前所述，我们发现 OFF 神经节细胞的反应阈值被士的宁升高。令人惊讶的是，士的宁并没有升高任何亚型的 OFF 锥状双极细胞的反应阈值。相反，在每个 OFF 锥状双极细胞亚型中，士的宁抑制了紧张性 Glycinergic 抑制，而不改变反应阈值。与士的宁缺乏影响一致，我们发现，在黑暗中 OFF 锥状双极细胞的主要输入是兴奋性的，兴奋性输入的反应阈值因亚型而异。因此，在暗适应的小鼠视网膜中，OFF 神经节细胞的高绝对敏感性不能用 OFF 锥状双极细胞的信号传递来解释。
