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.