Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons

Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons. J. Neurophysiol. 83: 1039-1051, 2000. Most of our knowledge about insect calcium currents is derived from studies on cultured or dissociated somata. So far, only little data on calcium currents are available for: neurons including their dendritic and presynaptic structures. Here we combined the switched-electrode voltage-clamp technique with optical recording using calcium-sensitive dyes in identified fly visual interneurons in vivo to characterize the voltage dependence and dynamics of calcium currents quantitatively and in a spatially resolved way. For all three cell types considered, i.e., centrifugal horizontal (CH), horizontal system (HS), and vertical system (VS) cells, the activation curve is rather flat and covers a voltage range from -60 to -20 mV in dendritic as well as presynaptic areas of the cells. The calcium increase is fastest for CH cells with a time constant of similar to 70 ms. In HS and VS cells, the time constant amounts to 400-700 ms. The calcium dynamics as determined in different regions of the cells are similar except for a small segment between the axon and the dendrite in HS and VS cells, where the calcium increase is significantly faster. In summary, the results show the existence of a low-voltage-activated calcium current with little or no inactivation in dendritic as well as presynaptic regions of fly lobula plate tangential cells.