Relatively little is known about the subcellular localization of low threshold Ca2+ channels (T-channels) in the brain. Using immunocytochemical labeling and preembedding immunoperoxidase and silver-enhanced immunogold electron microscopy, we localized T-channel subunit Cav3.3 in rodent cerebral cortex and thalamus. Double immunofluorescent staining demonstrated that Cav3.3-labeled neurons in cerebral cortex are a subgroup of GABAergic interneurons that coexpress calbindin and in half of the cases parvalbumin. In the thalamus, virtually all reticular nucleus (RTN) neurons were immunopositive for Cav3.3, while neurons in dorsal thalamic nuclei were nonimmunoreactive. At the electron microscopic (EM) level, in cortical layers IV-V and RTN neurons, Cav3.3 immunoreactivity was mainly associated with membranes of dendrites but with some localization in cytoplasm. None was found in axon terminals. In cortex, ≈73% of immunogold particles were present in close proximity to synaptic contacts (<0.5 μm from the postsynaptic density), while 27% were distributed along membranes at extrasynaptic sites (>0.5 μm from the postsynaptic density). In RTN, ≈57% particles were evenly distributed along perisynaptic membranes and the remaining 43% of particles were diffusely localized at extrasynaptic membranes. The density of particles along the dendritic membranes of cortical neurons was 40% higher than in RTN neurons. These results suggest that Cav3.3 plays a role in regulating GABAergic neurons whose actions underlie thalamocortical rhythmicity.
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