Imaging of calcium transients associated with tonic and burst firing in thalamic neuron

Calcium channels, especially low-threshold channels, are important for the functioning of thalamic relay cells. In this work, the localization of calcium channels and the calcium dynamics associated with burst and tonic firing was studied in thalamic relay cells.;Calcium imaging was done on thin thalamic slices from neonatal rats (10-20 days old) using confocal microscopy. Both relay and GABAergic cells were studied. They could be clearly distinguished based on their anatomical position, morphological appearance and electrical properties. The quality of the voltage clamp was evaluated by examining the fluorescence-voltage relationship, and it was found to be sufficiently adequate on both the proximal and distal dendrites of the relay cells to allow the low- and high-threshold currents to be activated separately using a voltage protocol. Under voltage clamp conditions, calcium influx associated with activation of low- and high-threshold calcium channels has been seen at the soma and both proximal and distal dendrites. These results indicate that calcium influx is mediated through calcium channels located at these sites. Under current clamp conditions, calcium influx can be evoked in the dendrites by low-threshold spikes or a train of action potentials generated at the soma, and this indicates that burst or tonic firing can spread into the dendrites and activate the dendritic calcium channels. During rhythmic bursting induced at the soma, calcium spikes at the soma and dendrites can follow the bursts, while nuclear signals are distinct only at low bursting frequency. The elevation of (Ca$sp{2+}rbracksb{rm i}$ occurs only during rhythmic firing, and no continuous or sustained increase of (Ca$sp{2+}rbracksb{rm i}$ is seen, at any region, after the cessation of burst firing. The mean elevation of (Ca$sp{2+}rbracksb{rm i}$ is proportional to the burst frequency. The functional significance of these findings is discussed.