Morgans C W
Synaptic Biochemistry Group, Division of Neuroscience, John Curtin School of Medical Research, Canberra, Australian Capital Territory.
Immunol Cell Biol. 2000 Aug;78(4):442-6. doi: 10.1046/j.1440-1711.2000.00923.x.
The synapses of photoreceptors and bipolar cells in the retina are easily identified ultrastructurally by the presence of synaptic ribbons, electron-dense bars perpendicular to the plasma membrane at the active zones, extending about 0.5 microm into the cytoplasm. The neurotransmitter, glutamate, is released continuously (tonically) from these 'ribbon synapses' and the rate of release is modulated in response to graded changes in the membrane potential. This contrasts with action potential-driven bursts of release at conventional synapses. Similar to other synapses, neurotransmitter is released at ribbon synapses by the calcium-dependent exocytosis of synaptic vesicles. Most components of the molecular machinery governing transmitter release are conserved between ribbon and conventional synapses, but a few differences have been identified that may be important determinants of tonic transmitter release. For example, the presynaptic calcium channels of bipolar cells and photoreceptors are different from those elsewhere in the brain. Differences have also been found in the proteins involved in synaptic vesicle recruitment to the active zone and in synaptic vesicle fusion. These differences and others are discussed in terms of their implications for neurotransmitter release from photoreceptors and bipolar cells in the retina.
视网膜中光感受器与双极细胞的突触在超微结构上很容易通过突触带的存在来识别，突触带是位于活性区垂直于质膜的电子致密条带，向细胞质延伸约0.5微米。神经递质谷氨酸从这些“带状突触”持续（紧张性地）释放，其释放速率会根据膜电位的分级变化而调节。这与传统突触中动作电位驱动的递质释放爆发形成对比。与其他突触类似，神经递质在带状突触处通过突触小泡的钙依赖性胞吐作用释放。控制递质释放的分子机制的大多数成分在带状突触和传统突触之间是保守的，但已发现一些差异，这些差异可能是紧张性递质释放的重要决定因素。例如，双极细胞和光感受器的突触前钙通道与大脑其他部位的不同。在参与突触小泡募集到活性区的蛋白质以及突触小泡融合方面也发现了差异。本文将讨论这些差异以及其他差异对视网膜中光感受器和双极细胞神经递质释放的影响。
