Visualization of Calcium Influx through Channels that Shape the Burst and Tonic Firing Modes of Thalamic Relay Cells. Qiang Zhou, Dwayne W. Godwin, Donald M. O'Malley and Paul R. Adams. Department of Neurobiology and Behavior, Life Sciences Building, State University of New York at Stony Brook, Stony Brook, NY 11794-5230.
APStracts 4:0056N, 1997.
ABSTRACT
Thalamic neurons have two firing modes: "tonic" and "burst". During burst mode, both low-threshold (LT) and high-threshold (HT) calcium channels are activated, while in tonic mode, only the HT-type of calcium channel is activated. The calcium signals associated with each firing mode were investigated in rat thalamic slices using whole-cell patch clamping and confocal calcium imaging. Action potentials were induced by direct current injection into thalamic relay cells loaded with a fluorescent calcium indicator. In both tonic and burst firing modes, large calcium signals were recorded throughout the soma and proximal dendrites. To map the distribution of the channels mediating these calcium fluxes, LT and HT currents were independently activated using specific voltage-clamp protocols. We focused on the proximal region of the cell (up to 50 æm from the soma) because it appeared to be well clamped. For a voltage pulse of a given size, the largest calcium signals were observed in the proximal dendrites with smaller signals occurring in the soma and nucleus. This was true for both LT and HT signals. Rapid imaging, using 1-dimensional linescans, was used to more precisely localize the calcium influx. For both LT and HT channels, calcium influx occurred simultaneously throughout all imaged regions including the soma and proximal dendrites. The presence of sizable calcium signals in the dendrites, soma and nucleus during both firing modes, and the presence of LT calcium channels in the proximal dendrite where sensory afferents synapse, have implications for both the electrical functioning of relay cells and the transmission of sensory information to cortex. 

Received 18 October 1996; accepted in final form 5 February 1997.
APS Manuscript Number J835-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 20 February 1997