[Turtle isthmic complex of visual nuclei: immunohistochemical study of gamma-aminobutyric acid, choline acetyltransferase, calcium-binding proteins and cytochrome oxidase activity].

The distribution of the immunoreactivity for gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), calcium-binding proteins (CaBPr) and histochemistry of cytochrome oxidase activity (CO) was studied in turtles (Testudo horsfieldi, Emys orbicularis) isthmal complex of visual nuclei. Magnocellular nucleus (IMc) was shown to reveal mainly the strongly stained GABA-, parvalbumin (PV)-ir neurons and CO-positive cells, as well as variable both in number and degree of intensity of ChAT-, cal- bindin (CB)-, and calretinin (CR)-ir cells. After the local tracer injection into the optic tectum GABA-ir neurons containing also retrograde label were found in IMc. The most caracteristic of the parvocellular nucleus (IPc) was the content of strongly stained ChAT-ir neurons, dense GABA-ir and CO-active terminal fields, as well as the neurons variable by the amount and the degree of immunoreactivity for CaBPr and GABA. Principal similarity in these features in the turtle IMc and IPc and of those in the avian isthmal nuclei of the same name allows suggesting their homology and consequently the same participation in selective processing of the visual information flow. The comparison with lower vertebrates confirms the evolutionary conservatism of visual isthmal complex among vertebrates and the existence of its progressive differentiation in the process of evolution.