Cytoarchitectural distribution of calcium binding proteins in midbrain dopaminergic regions of rats and humans.

The present study compares the distribution of three calcium binding proteins, calbindin-D28k, calretinin, and parvalbumin, in the midbrain tegmentum of rats and humans. In order to compare the distributions of these proteins directly, the cytoarchitecture of this region was evaluated by using immunohistochemistry for tyrosine hydroxylase and substance P in serial sections in both transverse and horizontal planes. There was a high degree of homology in the cytoarchitecture of the three main dopaminergic regions identified. The A8 group was localised in the retrorubral fields, which extended rostrally into the midbrain reticular fields in the human. The A9 group corresponded to the substantia nigra, which was delimited by its dense substance P innervation. The heterogeneous A10 group, situated along the dorsal border as well as medial to the A9 group, comprised multiple nuclei. The distribution of calcium binding proteins was similar in both species, although a larger proportion of neurons contained these proteins in the rat. Calbindin-D28k was localised in neurons within A8 and A10 nuclei and within the caudomedial A9 region (and rostrolateral A9 in the rat only). Calretinin was localised in similar regions. In contrast, neurons containing parvalbumin were concentrated in the substantia nigra pars reticulata. The results suggest that few dopaminergic neurons receiving striatal input in the substantia nigra contain calcium binding proteins; rather, the nondopaminergic nigral neurons contain parvalbumin. Interestingly, dopaminergic neurons are more numerous in humans, whereas nondopaminergic neurons predominate in rats, which suggests that functional differences may exist between rats and humans.