Turtle isthmic complex of visual nuclei: Immunohistochemistry of gamma-aminobutyric acid, choline acetyltransferase, calcium-binding proteins and histochemistry of cytochrome oxidase activity

The immunoreactivity patterns of gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), calcium-binding protein (CaBPr) and the histochemical pattern of cytochrome oxidase (CO) activity were studied in the isthmic complex of visual nuclei of turtles ( Testudo horsfieldi, Emys orbicularis ). Magnocellular nucleus (IMc) was shown to contain mainly the intensely labeled GABA- and parvalbumin (PV)-immunoreactive and CO-positive neurons as well as the variable both in number and degree of immunorecativity (-ir) ChAT-, calbindin (CB)-, and calretinin (CR)-ir neurons. Local tracer injection into the optic tectum revealed in IMc the GABAir neurons co-localizing the retrograde label. The most characteristic feature of the parvocellular nucleus (IPc) was the presence of the intensely labeled ChAT-ir neurons, dense GABA-ir and CO-active terminal fields, as well as GABA- and CaBPr-ir neurons variable in number and degree of immunoreactivity. The data obtained further corroborate the identity revision of the reptilian isthmic complex of visual nuclei we have undertaken previously. A fundamental similarity of the IMc and IPc isthmic nuclei in turtles and birds suggests their homology and similar participation in selective processing of visual information flow. A comparison with lower vertebrates confirms the evolutionary conservatism of the visual isthmic complex among vertebrates and its progressive differentiation in the process of evolution.