Mechanisms of dendritic calcium signaling in fly neurons

We examined the mechanisms underlying dendritic calcium accumulation in lob ula plate tangential cells of the fly visual system using an in vitro prepa ration of the fly brain. Local visual stimulation evokes a localized calciu m signal in the dendrites of these cells in vivo. Here we show that a simil ar localized calcium accumulation can be elicited in vitro by focal iontoph oretic application of the cholinergic agonist carbachol. The calcium signal had at least two sources: first, voltage-dependent calcium channels contri buted to the carbachol-induced signal and were concentrated on the dendrite , the soma, and the terminal ramification of the axon. However, the dendrit ic calcium signal induced by carbachol stimulation was only weakly dependen t on membrane depolarization. The most likely explanation for the second, v oltage-independent part of the dendritic calcium signal is calcium entry th rough nicotinic acetylcholine receptors. We found no indication of second-m essenger or calcium-mediated calcium release from intracellular stores. In summary, the characteristic spatiotemporal calcium signals in the dendrites of lobula plate tangential cells can be reproduced in vitro, and result fr om a combination of voltage- and ligand-gated calcium influx.