Ca2+ clearance in visual motion-sensitive neurons of the fly studied in vivo by sensory stimulation and UV photolysis of caged Ca2+.
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.
Kurtz
Rafael
R
Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany. rafael.kurtz@uni-bielefeld.de
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002118:Calcium / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology / Q000528:radiation effects; D004175:Diptera; D008564:Membrane Potentials / Q000502:physiology / Q000528:radiation effects; D009039:Motion Perception / Q000502:physiology* / Q000528:radiation effects; D009474:Neurons / Q000502:physiology* / Q000528:radiation effects; D010775:Photic Stimulation / Q000379:methods; D010782:Photolysis / Q000528:radiation effects; D014466:Ultraviolet Rays
D002118:Calcium