SPATIAL-DISTRIBUTION OF CA2-CELL DENDRITES IN-VITRO - ROLE OF A TRANSIENT OUTWARD CURRENT( INFLUX IN TURTLE PURKINJE)

1. Intracellular recordings were made from Purkinje cells in a slice p reparation of the turtle cerebellum. Simultaneously, changes in [Ca2+] i in all regions of the cell were detected with high-speed fluorescenc e imaging of injected fura-2. Cells were stimulated either intrasomati cally or synaptically. In addition, the cells were polarized locally w ith an external electrical field aligned parallel to the soma-dendriti c axis. 2. The soma, smooth dendrites, and spiny dendrites displayed v oltage-dependent changes in [Ca2+]i. Changes in the somatic region wer e correlated with Na+ spike firing and local depolarization. Small [Ca 2+]i changes in the spiny dendrites were correlated with graded potent ials and larger changes with Ca2+ action potentials. Individual Ca2+ s pike transients sometimes occurred separately in different dendritic r egions demonstrating localized firing. 3. The amplitude and spatial ex tent of spike-related [Ca2+]i transients were increased with intrasoma tic depolarizing prestimulus membrane potentials and reduced by hyperp olarizing prestimulus potentials. This dependence and the latency to C a2+ spike activation were strongly reduced by 4-aminopyridine (4-AP). These results suggest that a transient A-like current regulates the ge neration of Ca2+ spikes and the localization of Ca2+ influx in turtle Purkinje cell dendrites. 4. Both electric field depolarization and int rasomatic depolarization affected the generation of Ca2+ spikes and [C a2+]i signals in a similar manner. Strong field stimulation could evok e focal depolarization at the tips of the spiny dendrites and cause lo cal Ca2+ spike generation near the pial surface. When both stimuli wer e used, their effects were additive. 5. Climbing fiber (CF) or paralle l fiber (PF) stimulation were associated with the generation of dendri tic Ca2+ transients. In some experiments the PF-induced Ca2+ transient s were confined to a small part of the spiny dendrites. The spatial di stribution and the amplitude of these transients were influenced by so matic depolarization or field stimulation in a manner similar to their effect on directly evoked Ca2+ spikes and consistent with the involve ment of a transient outward current in the control of the synaptically induced Ca2+ influx. 6. These results suggest that the intrinsic pota ssium conductances dynamically modulate spatial integration and influe nce the compartmentalization of Ca2+ spikes and [Ca2+]i changes in the dendrites.