Changes in calcium signaling during postembryonic dendritic growth in Manduca sexta.

Activity-dependent Ca(2+) influx plays crucial roles in adult and developing nervous systems through its influence on signal processing, synaptic plasticity, and neuronal differentiation. The responses to internal Ca(2+) elevations vary depending on the spatial distribution of Ca(2+) accumulation in different cell compartments. In this study, the mechanisms and the distribution of Ca(2+) accumulation are addressed by in situ Ca(2+) imaging of an identified insect motoneuron, MN5, at critical stages of postembryonic life. During metamorphosis of Manduca sexta, MN5 undergoes extensive dendritic regression followed by regrowth. The time course, amplitude, and distribution of Ca(2+) accumulation within MN5 change during development. During the initial stage of rapid dendritic growth and branching, dendritic growth cones are present, and voltage-dependent Ca(2+) currents are small. At this stage, activity-induced elevations of internal Ca(2+) are largest in the distal dendrites, suggesting that the density of voltage-gated Ca(2+) channels is highest in these regions. Later phases of dendritic growth are accompanied by the transient occurrence of prominent Ca(2+) spikes. Single Ca(2+) spikes cause robust Ca(2+) influx of similar amplitudes and time courses in all central compartments of MN5. The resting Ca(2+) levels also increase during development. Ca(2+)-induced Ca(2+) release from intracellular stores did not contribute to the elevations measured at either stage, although Ca(2+) stores are present in the dendrites. These developmental changes of the internal Ca(2+) signaling are consistent with a regulatory role for activity-dependent Ca(2+) influx in postembryonic dendritic growth.