Calcium-binding proteins label functional streams of the visual system in a songbird

The vertebrate nervous system has been shown to contain high concentrations of intracellular calcium-binding proteins, each of them with a restricted expression pattern in specific brain regions and specific neuronal subpopulations. Using immunohistochemical staining techniques, we analyzed the expression pattern of calbindin, calretinin and parvalbumin in visual brain areas of a songbird species, the zebra finch (Taeniopyga guttata). Here we show that the analyzed proteins are expressed in a complementary fashion within different brain substructures generally corresponding to functional subpathways of the avian visual system. In detail, calbindin is expressed in the brain structures that belong to the thalamofugal pathway, whereas parvalbumin-positive neurons are found in the brain structures that are part of the tectofugal visual pathway. Originally, the expression of calcium-binding proteins has been associated with specific morphological or neurochemical criteria of neurons. Our results suggest that their expression pattern also indicates a functional segregation of brain substructures linked to vision in the zebra finch brain. As the selective labeling of functional streams has also been shown for the visual system in mammalian species, function-selective expression of calcium-binding proteins might be a general feature of vertebrates.