Spatial distribution of synaptically activated sodium concentration changes in cerebellar Purkinje neurons.

The spatial distribution of Na(+)-dependent events in guinea pig Purkinje cells was studied with a combination of high-speed imaging and simultaneous intracellular recording. Individual Purkinje cells in sagittal cerebellar slices were loaded with either fura-2 or the Na+ indicator sodium binding benzofuran isophthalate (SBFI) with sharp electrodes or patch electrodes on the soma or dendrites. [Na+]i changes were detected in response to climbing fiber and parallel fiber stimulation. These changes were located both at the anatomically expected sites of synaptic contact in the dendrites and in the somatic region. The variation in time course of these [Na+]i changes in different locations implies that Na+ enters at the synapse and diffuses rapidly to locations of lower initial [Na+]i. The synaptically activated somatic [Na+]i changes probably reflect Na+ entry through voltage-sensitive Na+ channels because they were detected only when regenerative potentials were recorded in the soma. [Na+]i changes in response to antidromically or intrasomatically evoked Na+ action potentials also were confined to the cell body. These observations are in agreement with other evidence that Na+ spikes are generated in the somatic region of the Purkinje neuron and spread passively into the dendrites. Plateau potentials, evoked by depolarizing pulses to the soma or dendrites, caused [Na+]i changes only in the soma, indicating that the noninactivating Na+ channels contributing to this potential also were concentrated in this region. The climbing fiber-activated [Na+]i changes were blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that these changes were not due to direct stimulation of the Purkinje neuron or activation of metabotropic receptors. Direct depolarization of the soma or dendrites never caused dendritic [Na+]i increases, suggesting that the climbing fiber-activated [Na+]i changes in the dendrites are due to Na+ entry through ligand-gated channels. A climbing fiber-like regenerative potential could be recorded in the soma after anode break stimulation, parallel fiber activation, or depolarizing pulses to the soma. The [Na+]i changes evoked by all of these potentials were confined to the cell body region of the Purkinje cell. [Ca2+]i changes in the dendrites evoked by the anode break potential were small relative to climbing fiber-activated changes, suggesting that a Ca2+ spike was not evoked by this response. The anode break and directly responses were blocked by tetrodotoxin. These results suggest that the somatically recorded climbing fiber response is predominantly a Na(+)-dependent event, consisting of a few fast action potentials and a slower regenerative response activating the same channels as the Na+ plateau potential.