Spatial distribution of Ca2+ influx in turtle Purkinje cell dendrites in vitro: role of a transient outward current.

1. Intracellular recordings were made from Purkinje cells in a slice preparation of the turtle cerebellum. Simultaneously, changes in [Ca2+]i in all regions of the cell were detected with high-speed fluorescence imaging of injected fura-2. Cells were stimulated either intrasomatically or synaptically. In addition, the cells were polarized locally with an external electrical field aligned parallel to the soma-dendritic axis. 2. The soma, smooth dendrites, and spiny dendrites displayed voltage-dependent changes in [Ca2+]i. Changes in the somatic region were correlated with Na+ spike firing and local depolarization. Small [Ca2+]i changes in the spiny dendrites were correlated with graded potentials and larger changes with Ca2+ action potentials. Individual Ca2+ spike transients sometimes occurred separately in different dendritic regions demonstrating localized firing. 3. The amplitude and spatial extent of spike-related [Ca2+]i transients were increased with intrasomatic depolarizing prestimulus membrane potentials and reduced by hyperpolarizing prestimulus potentials. This dependence and the latency to Ca2+ spike activation were strongly reduced by 4-aminopyridine (4-AP). These results suggest that a transient A-like current regulates the generation of Ca2+ spikes and the localization of Ca2+ influx in turtle Purkinje cell dendrites. 4. Both electric field depolarization and intrasomatic depolarization affected the generation of Ca2+ spikes and [Ca2+]i signals in a similar manner. Strong field stimulation could evoke focal depolarization at the tips of the spiny dendrites and cause local Ca2+ spike generation near the pial surface. When both stimuli were used, their effects were additive. 5. Climbing fiber (CF) or parallel fiber (PF) stimulation were associated with the generation of dendritic Ca2+ transients. In some experiments the PF-induced Ca2+ transients were confined to a small part of the spiny dendrites. The spatial distribution and the amplitude of these transients were influenced by somatic depolarization or field stimulation in a manner similar to their effect on directly evoked Ca2+ spikes and consistent with the involvement of a transient outward current in the control of the synaptically induced Ca2+ influx. 6. These results suggest that the intrinsic potassium conductances dynamically modulate spatial integration and influence the compartmentalization of Ca2+ spikes and [Ca2+]i changes in the dendrites.