The calcium-binding protein calretinin is localized in a subset of interneurons in the rat cerebral cortex: a light and electron immunohistochemical study.

The distribution of the calcium binding protein, calretinin (CR) has been investigated immunohistochemically in the cerebral cortex of albino rats by light- and electron microscopy. At the light microscopical level the pattern of CR-immunoreactivity (ir) appeared very similar in all regions of the rat cerebral cortex. CR-immunoreactive cells were found sparsely in layer I to layer VI, and frequently also in the white matter of the corpus callosum. All CR-ir neurons revealed morphological characteristics of local interneurons. The calretinin positive interneurons could be grouped according to their laminar occurrence, dendritic arborization and the soma size into 5 cell type classes. Quantitative measurements were performed only in the visual cortex. CR-ir neurons were more frequent in the superficial layers II and III. In all other layers, CR-ir cells are sparsely distributed with no preferential laminar localization. At the electron microscopical level, CR-ir axonal boutons formed frequently symmetrical axo-dendritic contacts. In all animals we observed CR-ir axons forming also synaptses of asymmetrical type. In summary calretinin labelled an interneuronal subpopulation of the rat cerebral cortex, which seemed not to overlap in its distribution and labelled structures to those, containing the related calcium binding proteins parvalbumin and calbindin D-28k.