Robson J G, Maeda H, Saszik S M, Frishman L J
College of Optometry, University of Houston, Houston, TX 77204-2020, USA.
Vision Res. 2004 Dec;44(28):3253-68. doi: 10.1016/j.visres.2004.09.002.
(a) To examine the possibility that there is a threshold in the synaptic mechanism linking rods to rod bipolar cells that can reduce the transmission of continuous noise from the rods without blocking the transmission of any significant proportion of single-photon responses. (b) To estimate the level of this threshold and the amplitude of the continuous noise which it can serve to reduce. (c) To identify the location of the threshold mechanism in the rod to rod bipolar cell pathway.
Corneal electroretinogram recordings were made from dark-adapted mice anesthetized with ketamine/xylazine after inner-retinal components had been suppressed to isolate PII, the response of depolarizing bipolar cells. Suppression was achieved by intravitreal injections of GABA, TTX, or in Cx36 KO animals by crushing the optic nerve and waiting for ganglion cells to degenerate.
All energy-scaled records of isolated PII obtained with ganzfeld stimuli that gave rise to much less than one photoisomerization (R*) per rod (0.01-0.2 R*/rod), had an essentially identical waveform. Stronger stimuli caused a reduction in the peak amplitude of energy-scaled records (saturation) and stimuli strong enough to produce multiple isomerizations in individual rods resulted in a shortening of the response latency and an increase of the energy-scaled amplitude at early times (supralinearity). The shape of the rising edge of isolated PII changed with flash energy in a way that was consistent with the existence of a synaptic threshold whose level was less than one tenth of the amplitude of single-photon signals and a continuous noise whose rms amplitude was even less than this. However, when measured at the time of the peak, the amplitude of PII increased linearly in proportion to stimulus energy from the very lowest levels up to the point where there was, on average, 0.2 R*/rod.
There is a threshold nonlinearity operating at the output of the rod to rod bipolar cell synapse that can usefully reduce the transmission of continuous rod noise without significantly affecting the transmission of single-photon signals. This nonlinearity does not affect the overall linear function of the rod pathway at levels at which it is effectively operating in a photon-counting mode.
（a）研究在连接视杆细胞与视杆双极细胞的突触机制中是否存在一个阈值，该阈值能够减少来自视杆细胞的连续噪声的传递，同时又不阻断任何显著比例的单光子反应的传递。（b）估计该阈值的水平以及它能够减少的连续噪声的幅度。（c）确定阈值机制在视杆细胞到视杆双极细胞通路中的位置。
在用氯胺酮/赛拉嗪麻醉的暗适应小鼠上记录角膜视网膜电图，在视网膜内成分被抑制以分离出PII（去极化双极细胞的反应）后进行记录。通过玻璃体内注射GABA、TTX来实现抑制，或者在Cx36基因敲除动物中通过压迫视神经并等待神经节细胞退化来实现抑制。
用全视野刺激获得的所有分离的PII的能量标度记录，这些刺激在每个视杆细胞中引起的光异构化（R*）远少于一个（0.01 - 0.2 R*/视杆细胞），其波形基本相同。更强的刺激导致能量标度记录的峰值幅度降低（饱和），而强度足以在单个视杆细胞中产生多次异构化的刺激会导致反应潜伏期缩短以及早期能量标度幅度增加（超线性）。分离的PII上升沿的形状随闪光能量而变化，这种方式与存在一个突触阈值一致，该阈值水平小于单光子信号幅度的十分之一，并且存在一种连续噪声，其均方根幅度甚至更小。然而，在峰值时刻测量时，PII的幅度从非常低的水平到平均每个视杆细胞有0.2 R*的点与刺激能量成线性比例增加。
在视杆细胞到视杆双极细胞突触的输出端存在一个阈值非线性，它可以有效地减少连续视杆细胞噪声的传递，而不会显著影响单光子信号的传递。在其以光子计数模式有效运作的水平上，这种非线性不会影响视杆细胞通路的整体线性功能。
