Calcium transients evoked by climbing fiber and parallel fiber synaptic inputs in guinea pig cerebellar Purkinje neurons

Abstract

1. Calcium transients related to climbing fiber (CF) and parallel fiber (PF) synaptic potentials were recorded from Purkinje cells in guinea pig cerebellar slices. Transients were measured using either absorbance changes of arsenazo III or fluorescence changes of fura-2, which were injected into individual cells in the slice. 2. All-or-none somatically recorded CF potentials elicited by white matter stimulation had all-or-none Ca transients. These signals began with a delay of > or = 2 ms from the start of the electrically recorded synaptic potential. The recovery time of CF-induced arsenazo III absorbance transients was < 50 ms in the fine dendrites in conditions that minimized the effects of dye buffering. 3. Ca2+ entry through voltage-gated Ca channels opened by Ca action potentials was the dominant source of the rise in [Ca2+]i after CF activation. There was no significant change in [Ca2+]i corresponding to the plateau potential that followed the large CF response. 4. The appearance and amplitude of distal CF-evoked Ca signals was more variable than proximal signals, suggesting that CF potentials do not reliably spread to the fine distal dendrites. The distal transient could be enhanced by intrasomatic depolarizing pulses, suggesting that it was a property of the postsynaptic membrane and not the presynaptic side of the CF synapse that was responsible for this variability. 5. Parallel fiber responses were evoked by electrical stimulation near the pial surface. Graded synaptic potentials and related Ca transients were reversibly blocked by 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Small synaptic potentials induced small, localized Ca transients. With increasing stimulus intensity, the PF electrical response developed a regenerative component. Larger dendritic Ca transients were detected corresponding to this component. Ca transients evoked by the regenerative responses had the same rapid rise times and fall times as those related to somatically stimulated Ca action potentials, suggesting that they also were due to Ca2+ entry through voltage-sensitive channels. 6. During trains of PF responses, we observed an increase in the spatial extent of related Ca transients. This effect could be modulated by changes in the resting potential, suggesting that the same intrinsic mechanism was affecting the spread of both CF and PF signals.