Spatial Distribution of Synaptically Activated Sodium Concentration Changes
in Cerebellar Purkinje Neurons.
Callaway, Joseph C. and William N. Ross.
Department of Physiology, New York Medical College, Valhalla, NY 10595,
TEL: 901-448-8497, FAX: 901-448-7193.
APStracts 3:0242N, 1996.
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 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 since they were only detected
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 non-inactivating Na + channels
contributing to this potential also were concentrated in this region. The
climbing fiber activated [Na + ] i changes were blocked by the AMPA receptor
antagonist CNQX, 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
following 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. [Ca 2+ ] i changes
in the dendrites evoked by the anode break potential were small relative to
climbing fiber activated changes, suggesting that a Ca 2+ spike was not evoked
by this response. The anode break and directly responses were blocked by TTX.
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.
Received 22 May 1996; accepted in final form 6 September 1996.
APS Manuscript Number J408-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996