Calcium-binding proteins and cytochrome oxidase activity in the turtle optic tectum with special reference to the tectofugal visual pathway

Using immunohistochemistry and a tracer technique we investigated the distribution in the optic tectum of turtles (Emys orbicularis and Testudo horsfieldi) of the calcium-binding proteins (CaBPr) parvalbumin (PV), calbindin (CB) and calretinin (CR) before and after labeling of the nucleus rotundus (Rot) with horseradish peroxidase. The optic tectum activity of the cytochrome oxidase (CO) was studied in parallel. In the principal link of the tectofugal visual pathway (central gray layer, SGC) in both chelonian species, the sparse PV-ir as well as CB- and CR-ir neurons were found significantly varying both in number and the intensity of immunoreactivity of their bodies and dendrites. In contrast, the superficial (SGFS) and deeper periventricular (SGP) tectal layers comprised numerous cells immunoreactive to all three CaBPr in different proportions. Only few retrogradely labeled tectorotundal SGC neurons expressed PV, CB or CR. The very large PV-ir neurons in SGC and SAC were not retrogradely labeled; morphologically they matched the efferent neurons with descending projections. SGC neurons of two chelonian species differed in the level of CO activity. Intense immunoreactivity to all three CaBPr and high CO activity were detected in both species in SGFS neuropil with some differences in sublaminar distribution patterns. The peculiarities of the CaBPr and CO activity distribution patterns in different segments of SGC neurons are discussed as related to the laminar organization of the turtle tectum and its retinal innervation. It is suggested that in the projection tectorotundal SGC neurons the CaBPr are concentrated mainly in their distal dendrites that contact retinal afferents in the superficial retinorecipient tectal layer.