The isthmus-tegmentum complex in the turtle and rat: a comparative analysis of its interconnections with the optic tectum.

Injections of horseradish peroxidase, wheat germ agglutinin and various amino acids into the optic tectum in both the turtle and rat, and into the nucleus isthmi magnocellularis (IsM) in the turtle were used to analyse the connections of the isthmus-tegmentum complex. The connectivities and the selective retrograde transport properties in certain tectal pathways were taken as a basis to define more accurately isthmus-tegmentum complex subdivisions. There were several main findings. In the turtle the projection from the tectum to the IsM originated in the stratum griseum periventriculare, whereas the projection from the IsM to the tectum terminated in the superficial tectal layers (both projections homolateral). The terminations of the pathway from IsM to tectum were not uniformly distributed throughout the tectal surface; rather, alternating zones of high and low termination density along the lateral dimension were observed. The turtle nucleus isthmi parvocellularis, receiving a few tectal fibers and afferents from the ipsilateral IsM, gave rise to a bilateral tectal projection. Evidence was obtained for a crossed collicular projection to the rat parabigeminal nucleus (Pbg) in addition to the established uncrossed one. GABA was retrogradely transported from the optic tectum to the Pbg in the rat, and to the dorsolateral mesencephalic tegmentum and the IsM in the turtle. After glycine injections into the optic tectum, the dorsomedial peri-parabigeminal tegmentum was retrogradely labeled in the rat, and the IsM in the turtle. An attempt was made to outline the parallelism between the organizations of the isthmus-tegmentum complexes in the turtle, pigeon and rat. It was concluded that some basic features in the inter-connectivity of the isthmus-tegmentum complex and other parts of the visual system have been preserved in evolution, despite the apparent loss of the isthmo-retinal projection in mammals.