Calcium-binding proteins and cytochrome oxidase activity in the turtle optic tectum with special reference to the tectofugal visual pathway
An immunohistochemical study was carried out to reveal distribution of immunoreactivity (IR) against Ca-binding proteins (CaBPr): calbindin (Calb), calretinin (Calr), and parvalbumin (Parv) in pretectum of the Central Asian terrestrial turtles (Testudo horsfieldi) and the pond turtles (Emys orbicularis). A wide distribution of CaBPr-IR neurons and terminals with a significant overlapping was established in most of pretectal nuclei (Lm, Pd, Pe, Po, and ndCP) with some differences in density of IR elements for various proteins. The alternative distribution of Parv-IR (a high density of IR neurons) and Calb/Calr-IR (a most low content of IR neurons and terminals) was found in the non-retinorecipient nucleus Ptv. Comparison of data obtained in turtles with literature data obtained in other species of reptiles and mammals has revealed a significant variability in the CaBPr distribution in some, presumably homologous, pretectal nuclei among various representatives of amniotes. The conclusion is made that in the process of evolution of amniotes, apart from conservatism of CaBPr-IR characteristics, there occurred their plastic rearrangements due to morpho-functional specialization of pretectal nuclei.