The relationship of calbindin-containing neurons with substance P, Leu-enkephalin and cholecystokinin fibres: An immunohistochemical study in the rat thalamus

In the rat thalamus, immunoreactivity for the calcium binding protein calbindin (Cb) is mostly confined to neuronal cell bodies, sometimes revealing proximal dendrites, of the midline, intralaminar and posterior regions. Substance P (SP)-, cholecystokinin (CCK)- and Leu-enkephalin (L-ENK)-immunoreactive (ir) elements in the thalamus are fibre-like structures, intermingled with punctate elements probably representing axonal arborizations and their synaptic boutons. These peptidergic fibres are unevenly distributed in several thalamic domains, including the areas that contain Cb-ir neurons. The relationship between Cb-ir cell bodies and these three different peptidergic systems of thalamic innervation was studied with immunohistochemistry. Single-labelling experiments on adjacent sections and double immunostaining on the same section were performed. A considerable overlap between Cb-ir perikarya and SP-ir fibres was found in most thalamic nuclei. In particular, in the intralaminar nuclei and posterior complex. SP-ir punctate elements were frequently observed in close proximity to Cb-ir cell bodies and dendrites. On the other hand, no consistent topographical correspondence between Cb-ir perikarya and CCK- or L-ENK-ir fibres was evident. Altogether, the present data suggest a selective anatomical and, possibly, functional relationship between SP and Cb in at least a subpopulation of rat thalamic neurons.