Regional calcium regulation within cultured Drosophila neurons:: Effects of altered cAMP metabolism by the learning mutations dunce and rutabaga

The dunce (dnc) and rutabaga (rut) mutations of Drosophila affect a cAMP-dependent phosphodiesterase and a Ca2+ / CaM-regulated adenylyl cyclase, respectively. These mutations cause deficiencies in several learning paradigms and alter synaptic transmission, growth cone motility, and action potential generation. The cellular phenotypes either are Ca2+ dependent (neurotransmission and motility) or mediate a Ca2+ rise (action potential generation). However, interrelations among these defects have not been addressed. We have established conditions for fura-2 imaging of Ca2+ dynamics in the "giant" neuron culture system of Drosophila. Using high K+ depolarization of isolated neurons, we observed a larger, faster, and more dynamic response from the growth cone than the cell body. This Ca2+ increase depended on an influx through Ca2+ channels and was suppressed by the Na+ channel blocker TTX. Altered cAMP metabolism by the dnc and rut mutations reduced response amplitude in the growth cone while prolonging the response within the soma. The enhanced spatial resolution of these larger cells allowed us to analyze Ca2+ regulation within distinct domains of mutant growth cones. Modulation by a previous conditioning stimulus was altered in terms of response amplitude and waveform complexity. Furthermore, rut disrupted the distinction in Ca2+ responses observed between the periphery and central domain of growth cones with motile filopodia.