A Computational Study on the Role of Gap Junctions and Rod I<i>_h</i> Conductance in the Enhancement of the Dynamic Range of the Retina

Recent works suggest that one of the roles of gap junctions in sensory systems is to enhance their dynamic range by avoiding early saturation in the first processing stages. In this work, we use a minimal conductance-based model of the ON rod pathways in the vertebrate retina to study the effects of electrical synaptic coupling via gap junctions among rods and among AII amacrine cells on the dynamic range of the retina. The model is also used to study the effects of the maximum conductance of rod hyperpolarization activated current Ih on the dynamic range of the retina, allowing a study of the interrelations between this intrinsic membrane parameter with those two retina connectivity characteristics. Our results show that for realistic values of Ih conductance the dynamic range is enhanced by rod-rod coupling, and that AII-AII coupling is less relevant to dynamic range amplification in comparison with receptor coupling. Furthermore, a plot of the retina output response versus input intensity for the optimal parameter configuration is well fitted by a power law with exponent . The results are consistent with predictions of more theoretical works and suggest that the earliest expression of gap junctions along the rod pathways, together with appropriate values of rod Ih conductance, has the highest impact on vertebrate retina dynamic range enhancement.已有研究表明，感觉系统中的间隙连接（gap junctions）的功能之一，是通过避免初始处理阶段出现早期饱和，来提升系统的动态范围（dynamic range）。本研究采用脊椎动物视网膜（vertebrate retina）视杆细胞ON通路（ON rod pathways）的最小化基于电导的模型（conductance-based model），探究视杆细胞之间以及AII无长突细胞（AII amacrine cells）之间通过间隙连接形成的电突触耦合（electrical synaptic coupling），对视网膜动态范围的影响。本模型同时用于探究视杆细胞超极化激活电流Ih（rod hyperpolarization activated current Ih）的最大电导，对视网膜动态范围的影响，借此可分析该内在膜参数与前述两种视网膜连接特性之间的相互关联。研究结果表明，当Ih电导取生理合理数值时，视杆细胞间的耦合可提升视网膜动态范围；且相较于视杆细胞间的耦合，AII无长突细胞间的耦合对动态范围扩增（dynamic range amplification）的贡献相对有限。此外，针对最优参数配置得到的视网膜输出响应随输入强度变化的曲线，可通过带指数项的幂律分布（power law）进行良好拟合。本研究结果与更多理论研究的预测相符，同时表明：视杆通路中间隙连接的早期表达，配合视杆细胞Ih电导的适宜取值，对脊椎动物视网膜动态范围提升的影响最为显著。