VISUALIZATION OF CALCIUM INFLUX THROUGH CHANNELS THAT SHAPE SIZE BURST AND TONIC FIRING MODES OF THALAMIC RELAY CELLS

Thalamic neurons have two firing modes: ''tonic'' and ''burst.'' Durin g burst mode, both low-threshold (LT) and high-threshold (HT) calcium channels are activated, while in tonic mode, only the I-IT-type of cal cium channel is activated. The calcium signals associated with each fi ring mode were investigated in rat thalamic slices using whole cell pa tch clamping and confocal calcium imaging. Action potentials were indu ced 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 some and proximal dendrites. To map the distribution of the channels mediating these cal cium fluxes, LT and HT currents were independently activated using spe cific voltage-clamp protocols. We focused on the proximal region of th e cell (up to 50 mu m from the soma) because it appeared to be well cl amped. For a voltage pulse of a given size, the largest calcium signal s were observed in the proximal dendrites with smaller signals occurri ng in the soma and nucleus. This was true for both LT and HT signals. Rapid imaging, using one-dimensional linescans, was used to more preci sely localize the calcium influx. For both LT and HT channels, calcium influx occurred simultaneously throughout all imaged regions includin g the soma and proximal dendrites. The presence of sizable calcium sig nals in the dendrites, soma, and nucleus during both firing modes, and the presence of LT calcium channels in the proximal dendrite where se nsory afferents synapse, have implications for both the electrical fun ctioning of relay cells and the transmission of sensory information to cortex.