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Ca²⁺ Clearance in Visual Motion-Sensitive Neurons of the Fly Studied In Vivo by Sensory Stimulation and UV Photolysis of Caged Ca²⁺

Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, D-33501 Bielefeld, Germany

Submitted 31 October 2003; accepted in final form 2 March 2004

In motion-sensitive visual neurons of the fly, excitatory visual stimulation elicits Ca²⁺ accumulation in dendrites and presynaptic arborizations. Following the cessation of motion stimuli, decay time courses of the cytosolic Ca²⁺ concentration signals measured with fluorescent dyes were faster in fine arborizations compared with the main branches. When indicators with low Ca²⁺ affinity were used, the decay of the Ca²⁺ 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 Ca²⁺ concentration decline in thin branches compared with thick ones is that the velocity of Ca²⁺ 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 Ca²⁺ to systematically elicit spatially homogeneous step-like Ca²⁺ concentration increases of varying amplitude. Clearance of Ca²⁺ liberated by this method depended on branch size in the same way as Ca²⁺ accumulated during visual stimulation. Furthermore, the decay time courses of Ca²⁺ signals were only little affected by the amount of Ca²⁺ released by photolysis. Thus Ca²⁺ efflux via the outer membrane is likely to be the main reason for the spatial differences in Ca²⁺ clearance in visual motion-sensitive neurons of the fly.

This article has been cited by other articles:

				J. Kalb, M. Egelhaaf, and R. Kurtz Robust integration of motion information in the fly visual system revealed by single cell photoablation. J. Neurosci., July 26, 2006; 26(30): 7898 - 7906. [Abstract] [Full Text] [PDF]