Simultaneous fluorescence imaging and electrophysiologic recordings were used to investigate the Ca2+influx initiated by action potentials (APs) into dorsal cochlear nucleus (DCN) pyramidal cell (PC) and cartwheel cell (CWC) dendrites. Local application of Cd2+blocked Ca2+transients in PC and CWC dendrites, demonstrating that the Ca2+influx was initiated by dendritic Ca2+channels. In PCs, TTX eliminated the dendritic Ca2+transients when APs were completely blocked. However, the Ca2+influx could be partially recovered during an incomplete block of APs or when a large depolarization was substituted for the blocked APs. In CWCs, dendritic Ca2+transients evoked by individual APs, or simple spikes, were blocked by TTX and could be recovered during an incomplete block of APs or by a large depolarization. In contrast, dendritic Ca2+transients evoked by complex spikes, a burst of APs superimposed on a slow depolarization, were not blocked by TTX, despite eliminating the APs superimposed on the slow depolarization. These results suggest two different mechanisms for the retrograde activation of dendritic Ca2+channels: the first requires fast Na+channel-mediated APs or a large somatic depolarization, whereas the second is independent of Na+channel activation, requiring only the slow depolarization underlying complex spikes.
