SPATIAL-DISTRIBUTION OF SYNAPTICALLY ACTIVATED SODIUM CONCENTRATION CHANGES IN CEREBELLAR PURKINJE NEURONS

The spatial distribution of Na+-dependent events in guinea pig Purkinj e cells was studied with a combination of high-speed imaging and simul taneous 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 o r patch electrodes on the soma or dendrites. [Na+](i) changes were det ected in response to climbing fiber and parallel fiber stimulation. Th ese changes were located both at the anatomically expected sites of sy naptic contact in the dendrites and in the somatic region. The variati on in time course of these [Na+](i) changes in different locations imp lies 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+ chan nels because they were detected only when regenerative potentials were recorded in the soma. [Naf]i changes in response to antidromically or intrasomatically evoked Na+ action potentials also were confined to t he cell body. These observations are in agreement with other evidence that Na+ spikes are generated in the somatic region of the Purkinje ne uron and spread passively into the dendrites. Plateau potentials, evok ed by depolarizing pulses to the soma or dendrites, caused [Na+](i) ch anges only in the soma, indicating that the noninactivating Na+ channe ls contributing to this potential also were concentrated in this regio n. The climbing fiber-activated [Na+](i) changes were blocked by the l pha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antago nist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that these chang es were not due to direct stimulation of the Purkinje neuron or activa tion of metabotropic receptors. Direct depolarization of the soma or d endrites never caused dendritic [Na+](i) increases, suggesting that th e climbing fiber-activated [Na+](i) changes in the dendrites are due t o Na+ entry through Ligand-gated channels. A climbing fiber-like regen erative potential could be recorded in the soma after anode break stim ulation, 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 th e dendrites evoked by the anode break potential were small relative to climbing fiber-activated changes, suggesting that a Ca2+ spike was no t evoked by this response. The anode break and directly responses were blocked by tetrodotoxin. These results suggest that the somatically r ecorded 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.