Tachibana M, Kaneko A
Department of Information Physiology, National Institute for Physiological Sciences, Okazaki, Japan.
Proc Natl Acad Sci U S A. 1988 Jul;85(14):5315-9. doi: 10.1073/pnas.85.14.5315.
L-Glutamate is a leading candidate for the vertebrate photoreceptor transmitter. In addition to the signal transmission to second-order neurons, photoreceptors communicate with each other not only electrically but also chemically. In the present study, by using solitary turtle photoreceptors, we examined the possibility that L-glutamate mediates interreceptor communication. L-Glutamate evoked an inward current in all subtypes of photoreceptors voltage-clamped to the resting potential. The highest glutamate sensitivity was located at the axon terminal. Both stereoisomers of aspartate were effective, whereas kainate, quisqualate, N-methyl-D-aspartate, and D-glutamate were ineffective. The presence of Na+ was essential to response generation; even Li+ could not substitute for Na+. The relation between L-glutamate-induced current and the membrane voltage was strongly inward-rectifying. These results favor the hypothesis that the L-glutamate-induced response is generated by an electrogenic uptake carrier. However, L-glutamate-induced current was always accompanied by an increase in current fluctuations, a phenomenon commonly observed in ion channels but not expected for an uptake carrier. Although the underlying mechanism needs further elucidation, it seems likely that L-glutamate is a transmitter for communication between photoreceptors.
L-谷氨酸是脊椎动物光感受器递质的主要候选者。除了向二级神经元进行信号传递外，光感受器不仅通过电方式，还通过化学方式相互通讯。在本研究中，我们利用分离的海龟光感受器，研究了L-谷氨酸介导感受器间通讯的可能性。L-谷氨酸在钳制于静息电位的所有光感受器亚型中诱发内向电流。谷氨酸的最高敏感性位于轴突末端。天冬氨酸的两种立体异构体均有效，而 kainate、quisqualate、N-甲基-D-天冬氨酸和D-谷氨酸则无效。Na+的存在对于反应的产生至关重要；即使Li+也不能替代Na+。L-谷氨酸诱导的电流与膜电压之间的关系呈强烈内向整流。这些结果支持这样一种假说，即L-谷氨酸诱导的反应是由生电摄取载体产生的。然而，L-谷氨酸诱导的电流总是伴随着电流波动的增加，这是在离子通道中常见的现象，但对于摄取载体来说是不预期的。尽管其潜在机制需要进一步阐明，但L-谷氨酸似乎是光感受器之间通讯的一种递质。
