Dendritic Ca transients evoked by action potentials in rat dorsal cochlear nucleus pyramidal and cartwheel neurons Running title: Dendritic Ca transients in DCN neurons

Simultaneous fluorescence imaging and electrophysiologic recordings were used to investigate the Ca influx initiated by action potentials (APs) into dorsal cochlear nucleus (DCN) pyramidal cell (PC) and cartwheel cell (CWC) dendrites. Local application of Cd blocked Ca transients in PC and CWC dendrites, demonstrating that the Ca influx was initiated by dendritic Ca channels. In PCs, TTX eliminated the dendritic Ca transients when APs were completely blocked. However, the Ca 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 Ca 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 Ca transients evoked by complex spikes, a burst of APs superimposed upon a slow depolarization, were not blocked by TTX, despite eliminating the APs superimposed upon the slow depolarization. These results suggest two different mechanisms for the retrograde activation of dendritic Ca 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.