Imaging of Ca2+ dynamics within the presynaptic terminals of salamander rod photoreceptors

Steele, Ernest C., Jr., Xiaoming Chen, P. Michael Iuvone, and Peter R. MacLeish. Imaging of Ca2+ dynamics within the presynaptic terminals of salamander rod photoreceptors. J Neurophysiol 94:4544-4553, 2005. First published August 17,2005;doi:10.1152/jn.01193.2004. Although the overall importance of Ca2+ as a mediator of cell signaling and neurotransmitter release has long been appreciated, the details of Ca2+ dynamics within the inner segments of vertebrate rod photoreceptors are just beginning to be elucidated. Even less is known regarding Ca2+ dynamics within the rod presynaptic terminal compartment. Using fura-2 to report changes in intracellular Ca2+, we imaged the responses of enzymatically dissociated salamander rod photoreceptors retaining intact axons and presynaptic terminals stimulated with a brief depolarizing puff of KCl (30 mM pipette concentration). In the vast majority of cells, the response was a large increase in Ca2+ levels in the terminal compartment, but not in the soma. In contrast, rods exhibited a substantial elevation in somatic Ca2+ levels when depolarized with a brief puff of 100 mM KCl (pipette concentration). These data are consistent with previously reported differences in Ca2+ buffering mechanisms within the somatic and terminal compartments. Additionally, they may reflect the presence of Ca2+ channels having distinct properties within the membranes of the two compartments. Consistent with this hypothesis, fluorescent immunocytochemistry using an antibody against the L-type Ca2+ channel Ca(v)1.2 (alpha 1C) subunit and semiquantitative confocal microscopy revealed a high concentration of immunoreactivity in the membranes of terminals of intact rods compared with the somata. Further investigations using enzymatically dissociated preparations of intact rod photoreceptors retaining their presynaptic terminals will allow further testing of these and other hypotheses regarding the compartmentalized regulation of Ca2+ dynamics within rod photoreceptors.