Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6058, USA.
J Neurosci. 2010 Apr 21;30(16):5533-43. doi: 10.1523/JNEUROSCI.4733-09.2010.
In the vertebrate visual system, ON cells respond to positive contrasts and OFF cells respond to negative contrasts, and thus both ON and OFF cells exhibit rectification. We investigated the retinal circuits by which the ON pathway rectifies the OFF pathway. White noise was projected onto an in vitro preparation of the mammalian retina and excitatory currents were recorded from retinal ganglion cells under whole-cell voltage clamp. Currents in OFF cells were more rectified than those in ON cells: thus, currents in ON cells were able to signal both positive and negative contrasts, but currents in OFF cells were virtually restricted to negative contrasts. Blocking signals in the ON pathway derectified currents in OFF ganglion cells, thus allowing them to be modulated by positive contrasts, indicating that the ON pathway normally rectifies currents in OFF ganglion cells. Such cross-rectification from ON to OFF pathways required intact glycinergic inhibition, indicating that a glycinergic amacrine cell, most likely the AII amacrine cell, allows the ON bipolar cell to hyperpolarize the OFF bipolar cell close to the threshold for transmitter release, thus rectifying excitatory currents in the OFF ganglion cell. Asymmetrical rectification of ON and OFF cells may be an adaptation to natural scenes that have more contrast levels below the mean than above. Thus, in order for ON and OFF pathways to encode an equal number of contrast levels, the ON cells must signal some negative contrasts.
在脊椎动物视觉系统中，ON 细胞对正对比度起反应，OFF 细胞对负对比度起反应，因此 ON 和 OFF 细胞都表现出整流特性。我们研究了使 ON 通路整流 OFF 通路的视网膜回路。白色噪声被投射到哺乳动物视网膜的体外制剂上，并在全细胞电压钳下从视网膜神经节细胞记录兴奋性电流。OFF 细胞中的电流比 ON 细胞中的电流更具整流性：因此，ON 细胞中的电流既能传递正对比度信号，也能传递负对比度信号，但 OFF 细胞中的电流实际上只限于负对比度信号。阻断 ON 通路中的信号会使 OFF 神经节细胞中的电流去整流，从而使它们能够被正对比度调制，表明 ON 通路通常会使 OFF 神经节细胞中的电流整流。这种从 ON 到 OFF 通路的交叉整流需要完整的甘氨酸能抑制，表明一种甘氨酸能无长突细胞，很可能是 AII 无长突细胞，允许 ON 双极细胞使 OFF 双极细胞去极化接近递质释放的阈值，从而整流 OFF 神经节细胞中的兴奋性电流。ON 和 OFF 细胞的不对称整流可能是对自然场景的一种适应，这些场景的对比度水平低于平均值的比高于平均值的多。因此，为了使 ON 和 OFF 通路能够编码相等数量的对比度水平，ON 细胞必须传递一些负对比度信号。
