Immunohistochemical study of the distribution of calcium binding proteins in the brain of a chondrostean (Acipenser baeri).

Chondrosteans represent an ancient lineage in ray-finned bony fishes and hence in jawed vertebrates. This immunohistochemical study in the brain of the Siberian sturgeon reports the neuronal distribution of three cytosolic calcium-binding proteins: calbindin-D28k (CB), calretinin (CR), and parvalbumin (PV). CB and CR are widely expressed in different neuron subsets distributed throughout the sturgeon brain. Studies using double immunofluorescence reveal a wide co-distribution of CB and CR in the brain nuclei but scarce co-localization at cellular level. In the forebrain, CR- and CB-immunoreactive (ir) populations were observed in the olfactory bulbs, in pallial and subpallial telencephalic areas, and in some diencephalic nuclei. CR-ir cells were also observed in the posterior tubercle and CB-ir cells in the preglomerular complex. At midbrain and hindbrain basal levels, CB-ir and CR-ir cell bodies were mainly distributed in periventricular areas. In the cerebellum, CB and CR cells were co-localized in some granular cell subsets in laterodorsal and dorsolateral regions, and in some Purkinje-like cells. CB-ir and CR-ir fibers were mainly observed in the olfactory bulbs, hypothalamus, and habenula, and in fiber tracts that coursed in the optic tectum and through the mesencephalic and rhombencephalic basal areas. With regard to PV, the sturgeon brain showed a rather limited distribution of PV-ir perikarya and fibers. Thus, CR, CB, and PV allowed the identification of subpopulations of neurons not distinguished on the basis of cytoarchitecture alone, which provided a better understanding of the anatomical organization of the sturgeon brain. These results reveal numerous shared features with teleosts, but also important differences.