Low‐threshold calcium channel subunit Ca v 3.3 is specifically localized in GABAergic neurons of rodent thalamus and cerebral cortex

Relatively little is known about the subcellular localization of low threshold Ca 2+ channels (T‐channels) in the brain. Using immunocytochemical labeling and preembedding immunoperoxidase and silver‐enhanced immunogold electron microscopy, we localized T‐channel subunit Ca v 3.3 in rodent cerebral cortex and thalamus. Double immunofluorescent staining demonstrated that Ca v 3.3‐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 Ca v 3.3, while neurons in dorsal thalamic nuclei were nonimmunoreactive. At the electron microscopic (EM) level, in cortical layers IV–V and RTN neurons, Ca v 3.3 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 Ca v 3.3 plays a role in regulating GABAergic neurons whose actions underlie thalamocortical rhythmicity. J. Comp. Neurol. 519:1181–1195, 2011. © 2010 Wiley‐Liss, Inc.