To understand the relationship between the propagation direction of action potentials and dendritic Ca2+ elevation, simultaneous measurements of intracellular Ca2+ concentration ([Ca2+]i) and intradendritic membrane potential were performed in the wind-sensitive giant interneurons of the cricket. The dendritic Ca2+ transients induced by synaptically-evoked action potentials had larger amplitudes than those induced by backpropagating spikes evoked by antidromic stimulation. The amplitude of the [Ca2+]i changes induced by antidromic stimuli combined with subthreshold synaptic stimulation was not different from that of the Ca2+ increases evoked by the backpropagating spikes alone. This result means that the synaptically activated Ca2+ release from intracellular stores does not contribute to enhancement of Ca2+ elevation induced by backpropagating spikes. On the other hand, the synaptically evoked action potentials were also increased at distal dendrites in which the Ca2+ elevation was enhanced. When the dendritic and axonal spikes were simultaneously recorded, the delay between dendritic spike and ascending axonal spike depended upon which side of the cercal nerves was stimulated. Further, dual intracellular recording at different dendritic branches illustrated that the dendritic spike at the branch arborizing on the stimulated side preceded the spike recorded at the other side of the dendrite. These results suggest that the spike-initiation site shifts depending on the location of the activated postsynaptic site. It is proposed that the difference of spike propagation manner could change the action potential waveform at the distal dendrite, and could produce synaptic activity-dependent Ca2+ dynamics in the giant interneurons.
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