Distribution of neurons immunoreactive for parvalbumin and calbindin in the somatosensory thalamus of the raccoon

The aim of this study was to assess the distribution of neurons immunoreactive for parvalbumin (PV), calbindin (CaBP), glutamic acid decarboxylase (GAD), and γ-aminobutyric acid (GABA) in the somatosensory thalamus of the raccoon and to compare these features to those of other species, especially primates. Immunohistochemistry was used to study the location of these neurons in the ventroposterior nucleus (VP), ventroposterior inferior nucleus (VPI), posterior group of nuclei (Po), and reticular nucleus (Rt). A consistent differential pattern of PV-positive (PV⁺) and CaBP-positive (CaBP⁺) cells was found in the somatosensory thalamus. Many PV⁺ neurons were observed in VP and Rt, but very few were found in VPI or Po. In contrast, CaBP⁺ neurons were distributed throughout VP, VPI, and Po but were very sparse or absent in Rt. In the VP nucleus, PV⁺ cells were distributed in clusters separated by interclusteral regions with a sparse distribution of PV⁺ cell bodies. The distributions of PV⁺ and CaBP⁺ cells tended to be complementary to each other in VP; regions with a high density of PV⁺ neurons had a low density of CaBP⁺ cell bodies. Double-labeling experiments revealed very few neurons in which PV and CaBP immunoreactivities were colocalized. Cells immunoreactive for GAD or GABA were found in PV-dense clusters of VP; fewer GABAergic neurons were present in the CaBP-dense interclusteral regions of VP and in VPI and Po. GAD⁺ and GABA⁺ neurons were most prominently distributed in Rt. We conclude that the distributions of PV⁺ and CaBP⁺ cell bodies in the raccoon somatosensory thalamus are very similar to those in primates. The density of GABAergic neurons in the somatosensory thalamus of the raccoon is less than that in the cat and monkey, but the relative distribution of GABAergic neurons in the different somatosensory nuclei is very similar to that in the cat and monkey. These results are discussed in relation to findings in other species and are related to the functions of lemniscal and nonlemniscal somatosensory pathways. J. Comp. Neurol. 388:120–129, 1997. © 1997 Wiley-Liss, Inc.