Pang Ji-Jie, Gao Fan, Wu Samuel M
Department of Ophthalmology, Baylor College of Medicine, Houston, TX, United States.
Front Cell Neurosci. 2021 Oct 14;15:722533. doi: 10.3389/fncel.2021.722533. eCollection 2021.
A chemical synapse is either an action potential (AP) synapse or a graded potential (GP) synapse but not both. This study investigated how signals passed the glutamatergic synapse between the rod photoreceptor and its postsynaptic hyperpolarizing bipolar cells (HBCs) and light responses of retinal neurons with dual-cell and single-cell patch-clamp recording techniques. The results showed that scotopic lights evoked GPs in rods, whose depolarizing Phase 3 associated with the light offset also evoked APs of a duration of 241.8 ms and a slope of 4.5 mV/ms. The depolarization speed of Phase 3 (Speed) was 0.0001-0.0111 mV/ms and 0.103-0.469 mV/ms for rods and cones, respectively. On pairs of recorded rods and HBCs, only the depolarizing limbs of square waves applied to rods evoked clear currents in HBCs which reversed at -6.1 mV, indicating cation currents. We further used stimuli that simulated the rod light response to stimulate rods and recorded the rod-evoked excitatory current (rdEPSC) in HBCs. The normalized amplitude (R/R), delay, and rising slope of rdEPSCs were differentially exponentially correlated with the Speed (all < 0.001). For the Speed < 0.1 mV/ms, R/R grew while the delay and duration reduced slowly; for the Speed between 0.1 and 0.4 mV/ms, R/R grew fast while the delay and duration dramatically decreased; for the Speed > 0.4 mV/ms, R/R reached the plateau, while the delay and duration approached the minimum, resembling digital signals. The rdEPSC peak was left-shifted and much faster than currents in rods. The scotopic-light-offset-associated major and minor cation currents in retinal ganglion cells (RGCs), the gigantic excitatory transient currents (GTECs) in HBCs, and APs and Phase 3 in rods showed comparable light-intensity-related locations. The data demonstrate that the rod-HBC synapse is a perfect synapse that can differentially decode and code analog and digital signals to process enormously varied rod and coupled-cone inputs.
化学突触要么是动作电位（AP）突触，要么是分级电位（GP）突触，二者不能兼具。本研究利用双细胞和单细胞膜片钳记录技术，探究了信号如何在视杆光感受器与其突触后超极化双极细胞（HBC）之间的谷氨酸能突触传递，以及视网膜神经元的光反应。结果显示，暗视光在视杆中诱发分级电位，其与光熄灭相关的去极化第3相也诱发了持续时间为241.8毫秒、斜率为4.5毫伏/毫秒的动作电位。视杆和视锥的第3相去极化速度（速度）分别为0.0001 - 0.0111毫伏/毫秒和0.103 - 0.469毫伏/毫秒。在记录的视杆和HBC对中，仅施加于视杆的方波去极化波峰在HBC中诱发了清晰的电流，该电流在 - 6.1毫伏时反转，表明是阳离子电流。我们进一步使用模拟视杆光反应的刺激来刺激视杆，并记录HBC中的视杆诱发兴奋性电流（rdEPSC）。rdEPSC的归一化幅度（R/R）、延迟和上升斜率与速度呈不同的指数相关（均<0.001）。对于速度<0.1毫伏/毫秒，R/R增加，而延迟和持续时间缓慢减少；对于速度在0.1至0.4毫伏/毫秒之间，R/R快速增加，而延迟和持续时间显著减少；对于速度>0.4毫伏/毫秒，R/R达到平台期，而延迟和持续时间接近最小值，类似于数字信号。rdEPSC峰值左移且比视杆中的电流快得多。视网膜神经节细胞（RGC）中与暗视光熄灭相关的主要和次要阳离子电流、HBC中的巨大兴奋性瞬态电流（GTEC）以及视杆中的动作电位和第3相显示出与光强度相关的类似位置。数据表明，视杆 - HBC突触是一个完美的突触，能够差异解码和编码模拟和数字信号，以处理极为多样的视杆和耦合视锥输入。
