Kurtz Rafael
Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany.
J Neurophysiol. 2004 Jul;92(1):458-67. doi: 10.1152/jn.01058.2003.
In motion-sensitive visual neurons of the fly, excitatory visual stimulation elicits Ca(2+) accumulation in dendrites and presynaptic arborizations. Following the cessation of motion stimuli, decay time courses of the cytosolic Ca(2+) concentration signals measured with fluorescent dyes were faster in fine arborizations compared with the main branches. When indicators with low Ca(2+) affinity were used, the decay of the Ca(2+) signals appeared slightly faster than with high affinity dyes, but the dependence of decay kinetics on branch size was preserved. The most parsimonious explanation for faster Ca(2+) concentration decline in thin branches compared with thick ones is that the velocity of Ca(2+) clearance is limited by transport mechanisms located in the outer membrane and is thus dependent on the neurite's surface-to-volume ratio. This interpretation was corroborated by UV flash photolysis of caged Ca(2+) to systematically elicit spatially homogeneous step-like Ca(2+) concentration increases of varying amplitude. Clearance of Ca(2+) liberated by this method depended on branch size in the same way as Ca(2+) accumulated during visual stimulation. Furthermore, the decay time courses of Ca(2+) signals were only little affected by the amount of Ca(2+) released by photolysis. Thus Ca(2+) efflux via the outer membrane is likely to be the main reason for the spatial differences in Ca(2+) clearance in visual motion-sensitive neurons of the fly.
在果蝇对运动敏感的视觉神经元中，兴奋性视觉刺激会引发树突和突触前分支中Ca(2+)的积累。运动刺激停止后，与主分支相比，用荧光染料测量的胞质Ca(2+)浓度信号在细分支中的衰减时间进程更快。当使用低Ca(2+)亲和力的指示剂时，Ca(2+)信号的衰减似乎比高亲和力染料时略快，但衰减动力学对分支大小的依赖性仍然存在。与粗分支相比，细分支中Ca(2+)浓度下降更快的最简洁解释是，Ca(2+)清除速度受位于外膜的转运机制限制，因此取决于神经突的表面积与体积比。通过对笼装Ca(2+)进行紫外闪光光解以系统地引发不同幅度的空间均匀阶梯状Ca(2+)浓度增加，这一解释得到了证实。通过这种方法释放的Ca(2+)的清除与视觉刺激期间积累的Ca(2+)一样，取决于分支大小。此外，Ca(2+)信号的衰减时间进程仅受光解释放的Ca(2+)量的轻微影响。因此，通过外膜的Ca(2+)外流可能是果蝇视觉运动敏感神经元中Ca(2+)清除存在空间差异的主要原因。
