Distribution of L-type calcium channels in rat thalamic neurones.

One major pathway for calcium entry into neurones is through voltage-activated calcium channels. The distribution of calcium channels over the membrane surface is important for their contribution to neuronal function. Electrophysiological recordings from thalamic cells in situ and after acute isolation demonstrated the presence of high-voltage activated calcium currents. The use of specific L-type calcium channel agonists and antagonists of the dihydropyridine type revealed an about 40% contribution of L-type channels to the total high-voltage-activated calcium current. In order to localize L-type calcium channels in thalamic neurones, fluorescent dihydropyridines were used. They were combined with the fluorescent dye RH414, which allowed the use of a ratio technique and thereby the determination of channel density. The distribution of L-type channels was analysed in the three main thalamic cell types: thalamocortical relay cells, local interneurones and reticular thalamic neurones. While channel density was highest in the soma and decreased significantly in the dendritic region, channels appeared to be clustered differentially in the three types of cells. In thalamocortical cells, L-type channels were clustered in high density around the base of dendrites, while they were more evenly distributed on the soma of interneurones. Reticular thalamic neurones exhibited high density of L-type channels in more central somatic regions. The differential localization of L-type calcium channels found in this study implies their predominate involvement in the regulation of somatic and proximal dendritic calcium-dependent processes, which may be of importance for specific thalamic functions, such as those mediating the transition from rhythmic burst activity during sleep to single spike activity during wakefulness or regulating the relay of visual information.