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

Kurtz, Rafael. Ca clearance in visual motion-sensitive neurons of the fly studied in vivo by sensory stimulation and UV photolysis of caged Ca . J Neurophysiol 92: 458–467, 2004; 10.1152/jn.01058. 2003. In motion-sensitive visual neurons of the fly, excitatory visual stimulation elicits Ca accumulation in dendrites and presynaptic arborizations. Following the cessation of motion stimuli, decay time courses of the cytosolic Ca concentration signals measured with fluorescent dyes were faster in fine arborizations compared with the main branches. When indicators with low Ca affinity were used, the decay of the Ca 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 concentration decline in thin branches compared with thick ones is that the velocity of Ca 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 to systematically elicit spatially homogeneous step-like Ca concentration increases of varying amplitude. Clearance of Ca liberated by this method depended on branch size in the same way as Ca accumulated during visual stimulation. Furthermore, the decay time courses of Ca signals were only little affected by the amount of Ca released by photolysis. Thus Ca efflux via the outer membrane is likely to be the main reason for the spatial differences in Ca clearance in visual motion-sensitive neurons of the fly.