Abstract
Low‐voltage‐activated (LVA) Ca^2+^ channels play a critical role in the generation of burst firing in the thalamus. Recently, three LVA Ca^2+^ channel isoforms (Ca~v~3.1, Ca~v~3.2, Ca~v~3.3) have been identified in the reticular thalamic nucleus (RE). Previous electrophysiological and modelling studies have suggested that kinetically different T‐type channels might be expressed in a compartmentalized manner in RE cells. However, their precise subcellular distribution has not been fully elucidated. Using light and electron microscopic (EM) immunocytochemistry, we investigated the subcellular expression pattern of Ca~v~3.1 and Ca~v~3.3 channel subunits in RE neurons of the cat. Fluorescent and peroxidase labelling demonstrated the presence of Ca~v~3.1 channel predominantly on the somata and proximal dendrites and Ca~v~3.3 channels on cell bodies. Quantitative immunogold localization disclosed that Ca~v~3.1 and Ca~v~3.3 isoforms showed 5.8‐ and 8.7‐fold higher density, respectively, in the cytoplasm compared with somatic plasma membrane. Density of Ca~v~3.1 isoform in the somatic plasma membrane was 2.21‐fold higher compared with Ca~v~3.3 subunit. In the dendritic plasma membrane, Ca~v~3.1 channel isoform was expressed throughout the entire dendritic tree. In contrast, Ca~v~3.3 isoform was absent from large‐caliber, presumably proximal dendritic segments. Quantitative comparison showed that the relative density of immunogold particles compared with dendritic surface was 8.9‐ and 14.8‐fold higher for Ca~v~3.1 and Ca~v~3.3, respectively, in small‐diameter dendrites than in large proximal dendritic segments or somata. Our results demonstrate a higher density of low‐threshold Ca^2+^ channels in distal dendrites and provide further evidence of the role of RE neuron dendrites in the generation of prolonged, low‐threshold spike bursts. © 2009 Wiley‐Liss, Inc.